__Shop A Cost function C(k) where k is the number of kilometers used__

C(k) = Cost per kilometer * k + Starting Charge

C(k) = 20k

With k = 7, we have:

C(7) = 20 * 7

C(7) = 140

__Shop B Cost function C(k) where k is the number of kilometers used__

C(k) = Cost per kilometer * k + Starting Charge

C(k) = 8k + 35

With k = 7, we have:

C(7) = 8 * 7 + 35

C(7) = 56 + 35

C(7) = 91

Bog should choose

**Shop B**since they have the better price for 7km