Answer:
$15
Step-by-step explanation:
Let d represent the cost of the dinner before tip. Then $15 = 0.20d, since 0.20 is the decimal fraction corresponding to 20%.
Solving for d, the cost of the dinner, we divide both sides of the equation $15 = 0.20d by .20, obtaining:
$15
d = ------------ = $75
0.20
The cost of the dinner before tip was $75.