The total that the shop spends on snow tires would be CT, the cost of a snow tire times the number of snow tires.
t+75
C = ---------
1-.05t
t+20
T = 500 ------
t+75
500 (t+75)(t+20)
So, CT = ------------------------
(1-.05t) (t+75)
Notice that the factor (t+75) appears in both the numerator and denominator so
500(t+20)
CT = ------------------
(1-.05t)