You have the following costs for the rent of a truck:
Cost of company A: $70 a day plus $0.20 per mile
Cost of company B: $40 a day plus $0.50 per mile
The previous cost can be written in the following algebraic form:
70 + 0.20x
40 + 0.50x
where x is the number of miles. In order to calculate the number of milles that makes the costs of both companies equal, you equal the two previous algebraic expression, just as follow:
70 + 0.20x = 40 + 0.50x you subtract 0.20x and 40 both sides
70 - 40 + 0.20x - 0.20x = 40 - 40 + 0.50x - 0.20x simplify
30 = 0.30x divide by 0.30 both sides
30/0.30 = 0.30x/0.30
100 = x
Hence, for 100 miles at day the costs of both companies are equal.