Private message from Aji as follows (couldn't reply directly as you've chosen not to receive private messages, so have posted reply for you here): Hi Aji It's ActEd's pretty way of integrating. For example, take Q5.12: They've taken a multiple of 30 giving: (0.3 + 1.5t) = 30(0.01 + 0.05t) This is so that when you integrate the whole thing, exp(1.68 - 0.01t-0.025t^2) gives you the right numbers so that the terms at the front cancel out neatly. If you don't spot the multiple that makes this works, don't worry - just do the integration "normally" and the solution will fall out of what you're doing. I hope this helps. Good luck with the study. Abi
