Question 242972
x varies inversely with y. 
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That says  {{{x=k/y}}}
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When x is 40, y is 50. 
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With that information we can determine k:

{{{x=k/y}}}

{{{40=k/50}}}

Multiply both sides by 50

{{{50*40=50*(k/50)}}}

{{{2000=k}}}

Now we must substitute that value 
of k into {{{x=k/y}}}

Now we have the complete equation.

{{{x=2000/y}}}

Now with that complete equation we can answer
the other part:
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What is y when x is 25?
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{{{x=2000/y}}}

Substitute 25 for x

{{{25=2000/y}}}

Multiply both sides by y

{{{y(25)=y(2000/y)}}}

{{{25y = 2000}}}

{{{y=2000/25}}}

{{{y=80}}}

Edwin</pre>