a) Let's say d is the number of days and C is the cost ($):
(1) Big Rigs: C = 3800 + 500d
if d = 6: C = 3800 + 500*6
= 3800 + 3000
= 6800
(2) Mr Digg: C = 5000 + 350d
if d = 6: C = 5000 + 350*6
= 5000 + 2100
= 7100
6800 < 7100, thus Big Rigs gives the better deal
b) Here we need to equate the two equations to see when the costs are the same:
3800 + 500d = 5000 + 350d
150d = 1200
d = 8, therefor it would take 8 days