difficulties with rates of change :( help please

[hello_kitty]

Hey Guys,
Im finding it really difficult to interpret graphs of rates of change, all i know is if its a straight line its a constant rate

I've been doign Ex 18 A and 18 B

and I'm unsure about some of the questions.
ive attached it as a PDF

one question is :

Find the rate of flow from the following taps in litres/min:
d A tap which takes 17 minutes 20 seconds to fill a 180 litre tank

Thanks Heaps!

[brightsky]

rate of flow is  180 litres in 17.3333333...minutes, which is 180/17.333333 litres in one minute, which is  135/13 litres a minute.