The charge C of a telephone company is partly constant and partly varies as the number of unit of call u.the cost of 50 unit is #2500 and the cost of 120 units is #3000. Find : the formular connecting C and u

Answer 1

C = mu + b
when u = 50, C = 2500
50u + b = 2500
when u = 120, C = 3000
120u + b = 3000
subtract the two equations:
70u = 500
u = 50/7
in 50u + b = 2500
2500/7 + b = 2500
b = 15000/7

Cost = (50/7)u + 15000/7

Answer 2

I didn't get it