Maths Olympiad Prep

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Algebra Difficulty 2.8 Junior Find the answer Canada

When two ants work together they can build an anthill in 24
minutes. When the bigger ant works alone, an anthill can be built in 14
minutes less than when the smaller ant works alone. How many minutes
does it take the smaller ant to build an anthill when working
alone?

A number or a short expression. Fractions can be typed as 3/2, and spacing doesn't matter.

Solution

Let xx be the number of
minutes that it would take the bigger ant to build an anthill alone and
let yy be the number of minutes that
it would take the smaller ant to build an anthill alone.

Since the bigger ant can build an anthill in xx minutes, the bigger ant builds 1x\dfrac{1}{x} anthills per minute.

Likewise, the smaller ant can build 1y\dfrac{1}{y} anthills per minute.

Thus, working together, the two ants build 1x+1y\dfrac{1}{x}+\dfrac{1}{y} anthills per
minute.

It is also given that it takes the two ants 2424 minutes to build an anthill together,
so this means they build 124\dfrac{1}{24} anthills per minute working
together.

Hence, we get the equation 1x+1y=124\dfrac{1}{x}+\dfrac{1}{y}=\dfrac{1}{24}.

Multiplying this equation through by 24xy24xy gives 24y+24x=xy24y+24x=xy.

From the other given condition, we get x=y14x=y-14, so we can substitute to get 24y+24(y14)=(y14)y24y+24(y-14) = (y-14)y.

Expanding and rearranging, this equation becomes y262y+336=0y^2-62y+336=0, which can be factored as
(y56)(y6)=0(y-56)(y-6)=0.

If y=6y=6, then x=8x=-8, which does not make sense since
xx must be positive. Therefore,
y=56y=56.

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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.