25% mark-up means that 25% of the original price was added to get the current price.
Remember a % is a FRACTION the denominator of which is 100.
original price + .25*original price = marked-up price
P(1.25)=M
P(1.25)=32.50
P=32.50/1.25
This is actually easier to do if you use a fraction instead of a decimal.
1.25=5/4
P*5/4=32.50
P=32.5*4/5=4*6.5=26