Feels like a very embarrassing question to ask but here goes.

I have a number of sales for a year (C3), and a percentage increase on the
previous year (C7). I need a formula to show the sales for that previous year.

eg 105 and 5% would be 100.

I know this is probably more a maths question than an excel question but I
can't seem to get anything to work - I keep coming up with 99.75 rather than
100.

Thanks for helping.