A parking garage charges $5.50 to park for 4 hours and $7.75 to park for 7 hours. If the cost is a linear function of a number of hours parked, what is the cost to park for 3 hours?

The cost per hour of parking is the gradient of the line and can be determined by:

(7.75 - 5.50) / (7 - 4)

= $0.75 per hour parking

5.50 = 0.75 x 4 + c

c = $2.50

The equation is:

y = 0.75x + 2.5

The cost for three hours (x = 3) is:

y = 0.75 (3) + 2.5

y = $4.75