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# 30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal

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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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06 Aug 2018, 10:20
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Question Stats:

90% (01:35) correct 10% (02:26) wrong based on 49 sessions

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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal to which of the following?

1. 79,900

2. 79,899

3. 79,879

4. 79,679

5. 79,569

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Joined: 02 Feb 2016
Posts: 34
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GMAT 1: 710 Q49 V38
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Re: 30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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06 Aug 2018, 10:23
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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798

= 99(799) + 798
= 99(799) + 799 -1
= 100(799) - 1
= 79900 - 1
= 79899
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Re: 30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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06 Aug 2018, 10:28
30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 can be written as
= 30(800-1) + 15(800-1) + 15(800-1) + 20(800-1) + 15(800-1) + 4(800-1) +798
= 24000-30 + 12000-15 + 12000-15 + 16000-20 + 12000-15 + 3200-4 + 798
= 79200-99+798
= 79899
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Re: 30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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06 Aug 2018, 11:45
Took too much time, 3:20
Am I supposed to waste more than 3 minutes on such time consuming examples?
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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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06 Aug 2018, 15:25
1
LevanKhukhunashvili wrote:
Took too much time, 3:20
Am I supposed to waste more than 3 minutes on such time consuming examples?

You are the lucky one. I have spent 4:11. That's okay, because some of questions will take less than 2 min, so that you will be able to compensate. Especially if you master DS, which is the real timesaver in Quant section.
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Joined: 22 May 2016
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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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07 Aug 2018, 18:16
AkshdeepS wrote:
30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal to which of the following?

1. 79,900

2. 79,899

3. 79,879

4. 79,679

5. 79,569

With numbers this cumbersome, start consolidating terms. A pattern or a step needed should strike.

30(799) + 15(799) + 15(799) + 20(799) + 15(799) + 4(799) + 798 =

Factor out 799
[(30+15+15+20+15+4)(799)] + 798 =

[(99)(799)] + 798

One more factor of 799 would make life a lot easier. Work with 798.
798 = 799 - 1

Now we have
(99)(799) + 799 - 1

Brackets make the next steps easier to see

[(99)(799) + (799)] - 1 =

There is an implied (*1) with the lone (799)
[(99)(799) + (799)(1)] - 1 =

Factor out (799) again
Take the two bolded factors and join them with the + operator in the equation
[(99 + 1) * (799)] - 1 =

(100)(799) - 1 =

79,900 - 1 =

79,899

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Re: 30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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07 Aug 2018, 18:59
LevanKhukhunashvili wrote:
Took too much time, 3:20
Am I supposed to waste more than 3 minutes on such time consuming examples?

It's a 45 second question at max if you find the pattern and apply it. FWIW, took me 58 seconds with the check to confirm accurate answer (more on that below). That's where the trick lies in all GMAT problems.

Quote:
30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal

Here is how I did it:

a(y) + b(y) + c(y) + d(y) is the same as (a+b+c+d)*y. In the case above: (30 + 15 + 15 + 20 + 15 + 4) * 799. Now you can either solve for 799 * 99 (the sum from before) and then add the 798 or you can do the following:

(30 + 15 + 15 + 20 + 15 + 4) * 799 + 799 - 1 --> 799*(99 + 1) - 1 --> 799 * 100 - 1 --> 79,900 - 1 --> 79,899. If you trust yourself, you could even get away without writing anything down on paper for this sort of issue, but why take that risk?

How do you check the answer?
1. Confirm the sum (30 + 15 + 15 + 20 + 15 + 4) is correct
2. Since the sum is 99 and you're multiplying by 99+1 and the subtracting 1, you know the last digit will be a 9. Reject A.
3. You're left with 4 options now. No check here. All end in 9. Just confirm the math quickly.
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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal  [#permalink]

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07 Aug 2018, 19:23
Hero8888 , I like that positive spirit! +1

LevanKhukhunashvili , hang in there. I can see how you would feel exasperated. Sometimes the shortcuts are just hard to see.

You both may have already seen this terrific post on timing strategies by @bb, founder and fearless leader of GMAT Club -- but if you have not, I highly recommend reading it.

Bon chance!
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30(799) + 15(799) +15(799) + 20(799) + 15(799) + 4(799) + 798 is equal   [#permalink] 07 Aug 2018, 19:23
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