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Math Expert V
Joined: 02 Sep 2009
Posts: 64071
A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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Difficulty:   45% (medium)

Question Stats: 74% (03:12) correct 26% (03:07) wrong based on 34 sessions

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A water tank has inlets of two types A and B. All inlets of type A when open, bring in water at the same rate. All inlets of type B, when open, bring in water at the same rate. The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open, and in 1 hour if 8 inlets of type A and 18 inlets of type B are open. In how many minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open?

A. 40
B. 42
C. 44
D. 46
E. 48

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Director  D
Joined: 25 Jul 2018
Posts: 712
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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Let x and y be the amount of water of each inlet brings into the water tank per minute via inlets of types of A and B, respectively.

$$30 (10x + 45y) =1$$
$$60 (8x + 18y )= 1$$
--> $$10x + 45y = \frac{1}{30}$$
--> $$4x + 9y = \frac{1}{120}$$
$$14x+ 54y = \frac{1}{30}+\frac{ 1}{120}= \frac{5}{120} = \frac{1}{24}$$
$$2 (7x + 27y) = \frac{1}{24}$$
--> $$48 (7x+ 27y) = 1$$

if 7 inlets of type A and 27 inlets of type B are open, $$48$$ minutes will the empty tank to get fulfilled.

Manager  B
Joined: 30 Jun 2019
Posts: 208
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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1=(10/a + 45/b)(1/2)
1=(8/a + 18/b)(1)

2 =10/a + 45/b
1 = 8/a + 18/b

4 =20/a + 90/b
5 = 40/a + 90/b
..
1 = 20/a --> a=20
..
2 = 10/20+45/b--> b=30

1 = (7/20+27/30)t
t=60/75
t=12/15

12/15*60 = 48min
Senior Manager  G
Joined: 21 Feb 2017
Posts: 469
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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fireagablast wrote:
1=(10/a + 45/b)(1/2)
1=(8/a + 18/b)(1)

2 =10/a + 45/b
1 = 8/a + 18/b

4 =20/a + 90/b
5 = 40/a + 90/b
..
1 = 20/a --> a=20
..
2 = 10/20+45/b--> b=30

1 = (7/20+27/30)t
t=60/75
t=12/15

12/15*60 = 48min

Hi could you explain why you've taken the equation as 10/a + 45/b and 8/a + 18/b. I am taking it as 10A+45B AND 8A+18B.. not able to get an answer though.
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Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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1
Bunuel wrote:
A water tank has inlets of two types A and B. All inlets of type A when open, bring in water at the same rate. All inlets of type B, when open, bring in water at the same rate. The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open, and in 1 hour if 8 inlets of type A and 18 inlets of type B are open. In how many minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open?

A. 40
B. 42
C. 44
D. 46
E. 48

Are You Up For the Challenge: 700 Level Questions

If we let a and b, respectively, be the number of minutes it takes a type A and a type B inlet to fill the tank completely, then 1/a and 1/b are the rates of a type A and a type B inlet, respectively. We can create the equations:

10(1/a) + 45(1/b) = 1/30 → 10/a + 45/b = 1/30

and

8(1/a) + 18(1/b) = 1/60 → 8/a + 18/b = 1/60

If we multiply the second equation by -2, we have: -16/a - 36/b = -1/30. Adding this to the first equation, we have:

-6/a + 9/b = 0

9/b = 6/a

9a = 6b

b = 9a/6 = 3a/2

Now, substitute this back to the first equation and solve for a:

10/a + 45/(3a/2) = 1/30

10/a + 90/(3a) = 1/30

Multiplying both sides by 30a, we have:

300 + 900 = a

1200 = a

Therefore, b = 3(1200)/2 = 1800.

Thus, the combined rate of 7 type A and 27 type B inlets is 7 x (1/1200) + 27 x (1/1800) = 7/1200 + 3/200 = 7/1200 + 18/1200 = 25/1200. Since time is the inverse of rate, the time is 1200/25 = 48 minutes.

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CEO  V
Joined: 03 Jun 2019
Posts: 2889
Location: India
GMAT 1: 690 Q50 V34 WE: Engineering (Transportation)
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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Bunuel wrote:
A water tank has inlets of two types A and B. All inlets of type A when open, bring in water at the same rate. All inlets of type B, when open, bring in water at the same rate. The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open, and in 1 hour if 8 inlets of type A and 18 inlets of type B are open. In how many minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open?

A. 40
B. 42
C. 44
D. 46
E. 48

Are You Up For the Challenge: 700 Level Questions

Given:
1. A water tank has inlets of two types A and B.
2. All inlets of type A when open, bring in water at the same rate.
3. All inlets of type B, when open, bring in water at the same rate.
4. The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open, and in 1 hour if 8 inlets of type A and 18 inlets of type B are open.

Asked: In how many minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open?

Let inlets A & B fill the water tank in x hours and y hours respectively

The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open

10/(2x) + 45/(2y) = 1

5/x + 45/2y = 1

The empty tank is completely filled in 1 hour if 8 inlets of type A and 18 inlets of type B are open.

8/x + 18/y = 1

5/x + 45/2y = 8/x + 18/y
3/x = 9/2y
6y = 9x
2y = 3x
y = 1.5x

8/x + 18/1.5x = 1
20/x = 1
x = 20
y = 30

In minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open

7/20 + 27/30 = 7/20 + 9/10 = 25/20 = 5/4

Time needed = 4/5 hours = 48 minutes

IMO E
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CEO  V
Joined: 03 Jun 2019
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Location: India
GMAT 1: 690 Q50 V34 WE: Engineering (Transportation)
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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Kritisood wrote:
fireagablast wrote:
1=(10/a + 45/b)(1/2)
1=(8/a + 18/b)(1)

2 =10/a + 45/b
1 = 8/a + 18/b

4 =20/a + 90/b
5 = 40/a + 90/b
..
1 = 20/a --> a=20
..
2 = 10/20+45/b--> b=30

1 = (7/20+27/30)t
t=60/75
t=12/15

12/15*60 = 48min

Hi could you explain why you've taken the equation as 10/a + 45/b and 8/a + 18/b. I am taking it as 10A+45B AND 8A+18B.. not able to get an answer though.

Hi Kritisood
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Kinshook Chaturvedi
Email: kinshook.chaturvedi@gmail.com
Senior Manager  G
Joined: 21 Feb 2017
Posts: 469
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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Kinshook wrote:
Kritisood wrote:
fireagablast wrote:
1=(10/a + 45/b)(1/2)
1=(8/a + 18/b)(1)

2 =10/a + 45/b
1 = 8/a + 18/b

4 =20/a + 90/b
5 = 40/a + 90/b
..
1 = 20/a --> a=20
..
2 = 10/20+45/b--> b=30

1 = (7/20+27/30)t
t=60/75
t=12/15

12/15*60 = 48min

Hi could you explain why you've taken the equation as 10/a + 45/b and 8/a + 18/b. I am taking it as 10A+45B AND 8A+18B.. not able to get an answer though.

Hi Kritisood

Understood, thanks a lot CEO  V
Status: GMATINSIGHT Tutor
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Posts: 3975
Location: India
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WE: Education (Education)
Re: A water tank has inlets of two types A and B. All inlets of type A whe  [#permalink]

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2
1
Bunuel wrote:
A water tank has inlets of two types A and B. All inlets of type A when open, bring in water at the same rate. All inlets of type B, when open, bring in water at the same rate. The empty tank is completely filled in 30 minutes if 10 inlets of type A and 45 inlets of type B are open, and in 1 hour if 8 inlets of type A and 18 inlets of type B are open. In how many minutes will the empty tank get completely filled if 7 inlets of type A and 27 inlets of type B are open?

A. 40
B. 42
C. 44
D. 46
E. 48

Are You Up For the Challenge: 700 Level Questions

The video explains a general principle (with derivation) and 2 step method to answer the problem

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ACCESS FREE GMAT TESTS HERE:22 FREE (FULL LENGTH) GMAT CATs LINK COLLECTION Re: A water tank has inlets of two types A and B. All inlets of type A whe   [#permalink] 12 Apr 2020, 22:28

# A water tank has inlets of two types A and B. All inlets of type A whe  