First let's find the cost of the dinner alone (without 8% sales tax) by solving the equation for x:
x + 0.08x = 32.40
1.08x = 32.40
x = 32.40/1.08
x = $30
Now calculate the 15% tip amount on the cost of the dinner alone:
(30)(0.15) = $4.50 (15% tip amount)