But then would you do 0.5x1/2? But that would equal 0.25? Sorry I know it sounds stupid but I'm actually confused how to get half a cent?

There are never stupid questions, ever. You can't know what you don't know, so don't be afraid to ask!

Recall that 100 cents make a dollar; this means that 1 cent is $0.01. Then, we halve it to get half a cent ie. $0.005

Does this make more sense?

2011-Q9-b-ii

Can someone please explain how did they get the answer as 18 litres? And like how do you know that you have to integrate (t)? Because I tried to sub 4 into both equations of liquid A and B and then subtract to get the answer as 4 litres? Help please!

You have to integrate here to find the volumes; you can't subtract the rates of change then multiply by the time taken. The rate of change given in the question is essentially just telling you how fast the water is coming out; it doesn't tell us anything about how much water is in the actual tank. And the answer is wrong; it's actually supposed to be 8 litres, not 18. There's a load of typos in the various sample answers, so just be wary of that