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Mr. Bridges can wash his car in 15 minutes, while his son Dave takes

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Mr. Bridges can wash his car in 15 minutes, while his son Dave takes [#permalink]

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New post 05 Mar 2018, 21:54
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A
B
C
D
E

Difficulty:

  5% (low)

Question Stats:

97% (00:36) correct 3% (01:03) wrong based on 88 sessions

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Mr. Bridges can wash his car in 15 minutes, while his son Dave takes twice as long to do the same job. If they work together, how many minutes will the job take them?

(A) 5

(B) 15/2

(C) 10

(D) 45/2

(E) 30

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Re: Mr. Bridges can wash his car in 15 minutes, while his son Dave takes [#permalink]

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New post 05 Mar 2018, 21:55
Bunuel wrote:
Mr. Bridges can wash his car in 15 minutes, while his son Dave takes twice as long to do the same job. If they work together, how many minutes will the job take them?

(A) 5

(B) 15/2

(C) 10

(D) 45/2

(E) 30


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New to the Math Forum?
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Collection of Questions:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Mr. Bridges can wash his car in 15 minutes, while his son Dave takes [#permalink]

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New post 05 Mar 2018, 23:21
While Mr. Bridges washes his car in 15 minutes, his son Dave takes twice as long(30 minutes)
Here, Mr. Bridges will do \(\frac{1}{15}\) of the work in a minute and Dave does \(\frac{1}{30}\) of the work in a minute

In a minute, working together, they would complete \(\frac{1}{15} + \frac{1}{30} = \frac{2+1}{30} = \frac{1}{10}\)th of the work in a minute.

Therefore, Mr. Bridges and Dave will take 10 minutes(Option C) to complete the work together.
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Re: Mr. Bridges can wash his car in 15 minutes, while his son Dave takes [#permalink]

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New post 06 Mar 2018, 10:17
Bridges will wash 2 cars in 30 mins while his son can wash one. In 30 mins both of them can wash 3 cars. So to wash 1 car they need 10 mins. The answer is C.
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Mr. Bridges can wash his car in 15 minutes, while his son Dave takes [#permalink]

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New post 07 Mar 2018, 16:17
1
Bunuel wrote:
Mr. Bridges can wash his car in 15 minutes, while his son Dave takes twice as long to do the same job. If they work together, how many minutes will the job take them?

(A) 5

(B) 15/2

(C) 10

(D) 45/2

(E) 30

Mr. B's time: 15 minutes
Dave's time is twice that: 30 minutes

When we have two separate times that A and B will take to finish a job, we can use a shortcut to find the time it will take them to finish together (units of time must be the same):

\(\frac{A*B}{A+B}\)

\(\frac{15*30}{15+30}=\frac{450}{45}=10\) minutes

Answer C
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Mr. Bridges can wash his car in 15 minutes, while his son Dave takes   [#permalink] 07 Mar 2018, 16:17
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