Maths Olympiad Prep

Track / Stage 6 / 350 of 400 #1350 of 1964

Problem 1350

National olympiad, first round
Algebra Difficulty 6.7 Find the answer

Volume A A equals one fourth of the sum of the volumes B B and C C, while volume B B equals one sixth of the sum of the volumes C C and A A.
Find the ratio of the volume C C to the sum of the volumes A A and B B.

A number or a short expression. Spacing and $ signs are ignored.

Official solution

1. We start with the given equations:
A=B+C4 A = \frac{B + C}{4}
B=C+A6 B = \frac{C + A}{6}

2. Rearrange the first equation to express CC in terms of AA and BB:
4A=B+C    C=4AB 4A = B + C \implies C = 4A - B

3. Rearrange the second equation to express CC in terms of AA and BB:
6B=C+A    C=6BA 6B = C + A \implies C = 6B - A

4. Now we have two expressions for CC:
C=4AB C = 4A - B
C=6BA C = 6B - A

5. Set the two expressions for CC equal to each other:
4AB=6BA 4A - B = 6B - A

6. Solve for AA in terms of BB:
4A+A=6B+B    5A=7B    A=75B 4A + A = 6B + B \implies 5A = 7B \implies A = \frac{7}{5}B

7. Substitute A=75BA = \frac{7}{5}B back into one of the original equations to find CC in terms of BB:
C=4AB=4(75B)B=285BB=285B55B=235B C = 4A - B = 4\left(\frac{7}{5}B\right) - B = \frac{28}{5}B - B = \frac{28}{5}B - \frac{5}{5}B = \frac{23}{5}B

8. We need to find the ratio of CC to A+BA + B:
A+B=75B+B=75B+55B=125B A + B = \frac{7}{5}B + B = \frac{7}{5}B + \frac{5}{5}B = \frac{12}{5}B

9. Therefore, the ratio of CC to A+BA + B is:
CA+B=235B125B=2312 \frac{C}{A + B} = \frac{\frac{23}{5}B}{\frac{12}{5}B} = \frac{23}{12}

The final answer is 2312\boxed{\frac{23}{12}}.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.