Maths Olympiad Prep

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Number theory Difficulty 2.7 Junior Find the answer Canada

If each of aa, bb and cc is a positive integer so that a+1b+1c=9011a+\dfrac{1}{b+\frac 1c}=\dfrac{90}{11},
then what is the value of a+b+ca+b+c ?

Pick one

Solution

Written as a mixed fraction, 9011=8211\dfrac{90}{11}=8\dfrac{2}{11}, and so
a+1b+1c=8+211a+\dfrac{1}{b+\frac1c}=8+\dfrac{2}{11}.

If a=8a=8, then 1b+1c=211\dfrac{1}{b+\frac1c}=\dfrac{2}{11}.

Since 211\dfrac{2}{11} has numerator
22, we rewrite the previous equation
as 2×12×(b+1c)=211\dfrac{2\times1}{2\times\left(b+\frac1c\right)}=\dfrac{2}{11}.

Both numerators are now equal to 22,
and so 2×(b+1c)=112\times\left(b+\frac1c\right)=11 or b+1c=112b+\dfrac1c=\dfrac{11}{2}.

Written as a mixed fraction, 112=512\dfrac{11}{2}=5\dfrac{1}{2}, and so b+1c=5+12b+\dfrac1c=5+\dfrac{1}{2}.

If b=5b=5, then 1c=12\dfrac{1}{c}=\dfrac{1}{2}, and so c=2c=2.

Therefore, a+1b+1c=8+15+12a+\dfrac{1}{b+\frac1c}=8+\dfrac{1}{5+\frac12}
and a+b+c=8+5+2=15a+b+c=8+5+2=15.

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Source: CEMC, University of Waterloo, licensed CC-BY-NC-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.