Question

Change c of a telephone company is partly constant and partly varies as the number of units call y the cost of 50 unti is 2500 and the cost of120 units 3000find the formular of connecting c and y when =4000

Answer 1

You will get a better response if your post is not in garbled English.
From what I can tell, the cost for 50 units is 2500, and cost of 120 units is 3000.
let c = my + b
for (50,2500), 2500 = 50m + b
for(120,3000), 3000 = 120m + b
subtract these .....
70m = 500
m = 50/7
and in 50m + b = 2500
50(50/7) + b = 2500
b = 15000/7

c = (50/7)y + 15000/7

once you have decided what "when =4000" means, plug into the appropriate variable, (my guess: you meant ... when c = 4000)