answer=$30.00
Increase in total spending = $10
+ (.75)($10)
+ (.75)(.75)($10)
+ (.75)(.75)($10)
+ ...
= $10 * (1 + .75 + .752 + .753 + ...)
----------------------------
|
(GEOMETRIC SERIES-- converges to a finite number,
according to a simple formula)
= $10 * [1/(1-.75)]
-----------
|
4
= $30