We want to increase by 15% so we want 100% plus 15% which means 115%
Since we want 115% lets make the calc:
65 pizzas ----------- 100%
x pizzas ------------- 115%
Multiply in cross
65 . 115 = 100.x
7475 = 100.x
x = 74,75
Since we cant sell 74 pizzas and 0,75 pizza we will sell 75 pizzas
So we need to sell 75 pizzas to employees receive the bonus