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19 May 2021, 01:04
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VeritasKarishma Could you please explain where I went wrong with this approach
As it is evident the strength of school A > School B.
I took School A strength as 140 and School B as 70 (since ratio 4+3 = 7. I choose multiples of 7)
40% of School A = 56 (girls)
60% of School B = 42 (girls)
on solving School A girls to School B girls are in 4:3.
boys in school A will be 140-56 = 84
boys in School B will be 70-42= 28
After transferring 20 students from School A to School B 64:48
Their difference is 16 and the ratio is 4:3
As it is evident the strength of school A > School B.
I took School A strength as 140 and School B as 70 (since ratio 4+3 = 7. I choose multiples of 7)
40% of School A = 56 (girls)
60% of School B = 42 (girls)
on solving School A girls to School B girls are in 4:3.
boys in school A will be 140-56 = 84
boys in School B will be 70-42= 28
After transferring 20 students from School A to School B 64:48
Their difference is 16 and the ratio is 4:3
19 May 2021, 20:36
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Hi BharatTj,
While TESTing VALUES can be used to solve lots of different Quant questions on Test Day, it's not a great approach here because there are so many ratios that you have to account for (including a "before" and "after" pair of ratios; this implies that an Algebraic approach will likely be necessary). In simple terms, you cannot choose any 'random' number you want to start with - and the proof is in your example:
You ended up with a ratio of 4:3, but the prompt states that the ratio is supposed to be 5:3. Your work actually proves that the starting number of children at the two Schools CANNOT be 140 and 70, respectively).
GMAT assassins aren't born, they're made,
Rich
While TESTing VALUES can be used to solve lots of different Quant questions on Test Day, it's not a great approach here because there are so many ratios that you have to account for (including a "before" and "after" pair of ratios; this implies that an Algebraic approach will likely be necessary). In simple terms, you cannot choose any 'random' number you want to start with - and the proof is in your example:
You ended up with a ratio of 4:3, but the prompt states that the ratio is supposed to be 5:3. Your work actually proves that the starting number of children at the two Schools CANNOT be 140 and 70, respectively).
GMAT assassins aren't born, they're made,
Rich
20 May 2021, 00:31
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BharatTj
You don't need to take numbers that are multiples of 7. 7 is the total number of girls in both schools together on the ratio scale, but it is not one of our parameters in the question. We just want that number of girls in each school should be multiples of 4 and 3 respectively. So stick to easier numbers such as 200, 100 and then number of girls as 80 and 60 respectively.
Note that when you assume numbers, they should fall in place such that after you transfer the 20 boys, their ratio must be 5:3 (as given). You get a different ratio with your numbers. It means your assumed numbers are not correct. You cannot know whether your assumed numbers are correct without calculating the ratio. Normally, when you have a value increase or decrease and not percentage (i.e. 20 boys transferred), assuming numbers could be tricky. You might need to adjust them around. If it seems cumbersome, algebra would be the way to go.
Often on GMAT, the numbers are quite simple. I took the numbers I mentioned while reading the question and figuring out the logic involved. Luckily, they worked. Else, I would have had to tweak them around depending on whether I was getting a lower or a higher ratio than expected.
