When you mark something up, you add the percent to 100%, chnage it to a decimal
and multiply be the old price.

When you mark something down, you subtract the percent from 100%, chnage it to
a decimal and multiply be the old price. 

Suppose the dress costs  $N

So we subtract 100%-25% getting 75%.  We change that to the decimal .75 and
multiply it by 5N and get $.75*N

So we subtract 100%-20% getting 80%.  We change that to the decimal .80 and
multiply it this time by $.75N and get $.80×.75*N = $.60N.  So it's the same as
multiplying by .60 which is 60%, which is what you would get if you subtracted
the discount percentage from 100%, which means the percent the dress was marked
down after both discounts is 100%-60% or 40%.

Edwin