let us assume that the number of pizzas he must deliver = x
The tip obtained for one pizza = $1.50
Total tips that he must earn = 20
So, we frame the equation as shown below:
1.50*x = 20
1.50x = 20
x = 20/1.5 = 13.333
So he must deliver 13.333 pizzas
Since he cannot deliver fractional pizzas, the number of pizzas he must deliver = 14 pizzas