1. $12,000 - $10,920 = $1080

$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.