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

Question

a.the formula connecting c and u.
b.u when c =#4000

Answer 1

c = mu + b
so, using the two given data points, you just need to solve
50m+b = 2500
120m+b = 3000
Now, having m and b, you can find u when c=4000

Or, even without knowing m and b, note that m grew by 70 when c grew by 500. So, if c grows another 1000, m will grow another 140.
So, u=260 when c=4000.