It is called being proportional,
Ex. so every three grapes you eat, i eat four
every 6 you eat, I eat 8, etc. etc.
Assuming that C is the hypotenuse...
The formula you want to use is:
a^2 + b^2 = c^2
We start by squaring 10, which is 100. 5^2 is 25. Now that we have the values, we can do the equation.
100+25=c^2 100+25=
Now, square root 
C= 11.180