Statistics question problem [Urgent]

[Joseph Potts]

I was doing 2009's paper 2 and I had a hard time solving this question:-

A school wished to estimate its examination costs per candidate for the coming year using the previous year’s fees.
For each candidate there was a basic fee of $20 plus an additional fee of $30 for each subject taken by that candidate. Using these figures, the school estimated that the cost per candidate would have a mean of $230 and a standard deviation of $90.
However, before the entries were made, the fees had been increased to $25 per candidate and $35 per subject.
Calculate the actual mean and standard deviation of the cost per candidate which the school will have to pay for its examination entries in the coming year.

I saw the examiner report and it gave the answer saying it had something to do with a scale factor. Can someone please explain to me the formula (and even write it so I know) so that I won't have this problem again.