Whose rating is higher?

Algebra Level 4

Jim and Bob are maths students who both use Brilliant.Their rating increases by ( P C 25 ) 2 (\frac{P-C}{25})^{2} when they solve a problem with a rating higher than their own rating,where C C is their current rating and P P is the rating of the problem they solved.When they get a problem wrong, their rating decreases by ( P C 25 ) 2 (\frac{P-C}{25})^{2} if the rating of the problem solved wrongly is less than their current rating and their rating decreases by 4 C P \frac{4C}{P} if the rating of the problem is more than their current rating. Both of their ratings in Algebra are 1600 1600 .This morning,Jim solved a problem with a rating of 1700 1700 .After that,he got a problem with a rating of 1591 1591 wrong.When Bob found out that Jim's rating had increased,he immediately started doing problems on Brilliant.Bob got a problem with a rating of 1500 1500 wrong.What is the minimum rating of the next problem he has to solve correctly if he only has time to solve one problem and he wants to beat Jim or make their ratings equal?

  • Assume that Brilliant ratings are rounded to the nearest integer.

  • Round your answer to the nearest integer.


The answer is 1723.

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1 solution

Tan Li Xuan
Mar 23, 2014

First,Jim solved a problem with rating 1700 1700 when his current rating is 1600 1600 ,so his rating increases by

( 1700 1600 25 ) 2 = ( 100 25 ) 2 = 4 2 = 16 (\frac{1700-1600}{25})^{2} = (\frac{100}{25})^{2} = 4^{2} = 16

Jim's rating is now 1616 1616 .Then, his rating drops by

( 1591 1616 25 ) 2 = ( 25 25 ) 2 = 1 (\frac{1591-1616}{25})^{2} = (\frac{-25}{25})^{2} = 1

So,Jim's rating is now 1615 1615 .When Bob got his first problem wrong,his rating drops by

( 1500 1600 25 ) 2 = ( 100 25 ) 2 = 4 2 = 16 (\frac{1500-1600}{25})^{2} = (\frac{-100}{25})^{2} = 4^{2} = 16

Bob's rating is 1600 16 = 1584 1600 - 16 = 1584 .To beat Jim,Bob needs to increase his rating by 1615 1584 = 31 1615 - 1584 = 31 .Then,we can construct an inequality

( P 1584 25 ) 2 31 (\frac{P-1584}{25})^{2} \geq 31

Taking the square root of both sides, we get

P 1584 25 5.5677643628...... \frac{P-1584}{25} \geq 5.5677643628......

Then,we multiply both sides by 25

P 1584 139.19410907...... P-1584 \geq 139.19410907......

Adding 1584 to both sides,we get

P 1723.19410907...... P \geq 1723.19410907......

So our answer is 1723 \boxed{1723} .

But 1723 is smaller than 1723.19410907......

Takeda Shigenori - 7 years, 2 months ago

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The problem states "Round your answer to the nearest integer".

Tan Li Xuan - 7 years, 2 months ago

the answer should be 1724 . . . not 1723 as P should be greater than 1723.19

Abhilash Jha - 7 years, 2 months ago

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Brilliant ratings are rounded to the nearest integer.If the problem rating is 1723 \boxed{1723} then Bob's rating is 1614.9136 1614.9136 which is rounded to 1615 1615 .

Tan Li Xuan - 7 years, 2 months ago

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