Let the original cost per person = C
So......the revenue collected is just 20C
When 10 more join, the cost per person is (C - 1.50)
And the revenue is 30 (C - 1.50)
And the revenue (cost for renting the bus) is the same in both cases....so
20C = (30) (C - 1.50) simplify
20C = 30C - 45 subtract 30C from both sides
-10C = - 45
C = 45/10 = 4.50
So.....the cost of renting the bus = 20 C = 20 * 4.50 = $90
