$12000n = $1080
n = .09
Thus it is a 9% decrease.
2. Some number a is the original price, so that:
.15(a) + a = $65.32
1.15a = $65.32
a = $56.80
To double check:
$56.80 x .15 = $8.52 <-- That is our tip
$56.80 + $8.52 = $65.32 <-- The price we had before.
So it checks out.
Basically I just used the percent equations:
(Original Value) x n = (New Value)
Which says that some percent n of the original value is the new value.