Question

The cost of making a dress is partly constant and partly varies with the amount of time it takes to make the dress.if the dress takes 3hrs to make,it cost #2700.if it takes 5hrs to make dress,it cost #3100.find the cost if it takes 1½hours to make the dress.

Answer 1

cost = a + bt, where a and b are constants and t is the time to make a dress in hours.
from your given:
a + 3b = 2700
a + 5b = 3100
subtract them
2b = 400
b = 200

now that you have b, find a from one of the equations.
given you the complete equation.
sub in t = 1.5 to get the cost