w/5=7/10 solve for w
Try to get all variables (letters) on one side, numbers on the other. Your problem is that the answer, w, is divided by 5 on the lhs. You must do the opposite (ie multiply by 5) throughout the expression (do it on both sides) to remove the offending 5 and hence get w = ....
Multiply both sides by 5
Do some cancellation on lhs and tidying up on rhs
Hope that helps.