For me it is easier to think in amounts of 10. For example, let's say something in the store has a price of $55.00 and has a markdown of 20%. Here's what I do: 10% of $55.00 equals $5.50 (you move the decimal point over one position to the left). You can verify this is correct by multiplying $5.50 x 100 = $55.00. I then double the amount of $5.50 to find the 20% which would be $5.50 + $5.50 = $11.00.

20% of $30.00 = (10%=$3.00) $3.00 + $3.00 = $6.00

40% of $30.00 = (10% of $30.00 = $3.00) so 40% = 4 x $3.00=$12.00

20% of $30.00 = (10%=$3.00) $3.00 + $3.00 = $6.00

40% of $30.00 = (10% of $30.00 = $3.00) so 40% = 4 x $3.00=$12.00