The Rivera family and the Cook family each used their sprinklers last summer. The water output rate for the Rivera family's sprinkler was 40 per hour. The water output rate for the Cook family's sprinkler was20 per hour. The families used their sprinklers for a combined total of 65 hours, resulting in a total water output of 2000. How long was each sprinkler used?

- "runner b is moving at a slower rate."

- "the graphs shown are graphs of distance(km) against time(min).

the slopes of this graphs are are given by:

slope = (change in y)/(change in x)

= (change in distance)/(change in time)

this is the formula for finding the speed. so, the slope of these graphs represent the speed of the runner.

taking each division in the graph to represent 1 on both axis:

runner a ⇒ speed = (2-0)/(1-0)

= 2/1

= 2 km/min

runner b ⇒ speed = (3-0)/(3-0)

= 3/3

= 1 km/min

this shows that   runner b is moving at a slower rate."