A: y=115+4(n/15) where n is the number of customers.
y=115+4n/15
B: An increase in the y intercept would mean the base number of pizzas they sold would be higher. The gradient would remain the same.
C: Say they sold an additional 4 for every 4 customers instead.
y=115+4(n/4)
y=115+4n/4
The gradient in this case would be 1.
I.e. the gradient would be different however the intercept would be the same as they are still selling the same number of pizzas.