Question

The cost of buying is partly constant and partly varies with number of shirt bought when the number of shirt is 5, the cost is 240, also ten shirt cost is 400 find the cost when 300 shirt were bought

Answer 1

c = cost
n =number
then
c = m * n + b
-----------------------
240 = 5 m + b
400 =10 m + b
--------------------- subtract
- 160 = - 5 m
m = 32
so c = 32 n + b

Answer 2

cost = a + bs, where a and b are constants, and s is the number of shirts.

a + 5b = 240
a + 10b = 400

I would subtract them, get b, and then a

you now have the full equation,
find cost when s = 300

Answer 3

400 = 32 (10) + b
b = 80
so
c = 32 n + 80
if n = 300
c = 32 * (300) + 80