Posted by
**Sara** on
.

Solve S=a/(1-r) for r

S = a/(1-r)

Multiply both sides by 1-r

S*(1-r) = [(1-r)*a]/(1-r)

Simplify

S*(1-r) = a

Multiply terms on the left.

S - S*r = a

Subtract S from both sides.

S-S*r - S = a - S

Simplify

-S*r = a - S

Multiply both sides by -1

S*r = -a+S

Divide both sides by S

S*r/S = (-a + S)/S

r = (S-a)/S

Check my work carefully.