if I varies inversely as r+4, and I=8 when r=3, find r when I=5

Question

help please this is hard 

Answer 1

I varies inversely as r + 4 means:  I = k/(r+4)

k is the constant of proportion

 

Plug in the initial given values of I=8 and r=3 to solve for k

 

8 = k/(3+4)

8 = k/7

56 = k

 

Now use the value of k to plug in and find the indicated value of r

from the equation I = 56/(r+4)

 

5 = 56/(r+4)

5(r+4) = 56

5r + 20 = 56

5r = 56-20

5r = 30

r = 6