To find the inverse, interchange the variables and solve for y. You should get
f^-1 (x)= -56/5 + 7x/5
If you want to keep it technical, the normal “/unit rate” for running is minutes per mile. You can use dimensional analysis like I did below to find the answer. You should get 11.45 min/mile or about 11 min 25 sec per mile
A(n)=ar^(n-1) and we can find the rate upon using the ratio of two points...
50/1250=1250r^2/1250r^0
1/25=r^2
r=1/5 so
a(n)=1250(1/5)^1=250
...
You could have also found the geometric mean which is actually quite efficient too...
The geometric mean is equal to the product of a set of elements raised to the 1/n the power where n is the number of multiplicands...in this case:
gm=(1250*50)^(1/2)=250