draw a graph with number of bottles on the x axis ( and number it 1 through 10 )
put price on the y axis and label it in increments of $1.50 ( 1.50, 3.00 , 4.50, 6.00, etc.
Then put dots on the intersection of price with the number of bottles and then draw a straight line through the dots.
for every 1 bottle of water sold the price increases by 1.50
so at x 1 y = 1.50
at x 2 y = 3.00
at x 3 y = 4.50
and so on