If you have no calculator, pretend like the decimals aren’t there then add them back depending on how many places you have after the decimal. for example: 2.5 times 2.5, if you pretend there are no decimals, you get 625, then you have to add the decimals back. since you have 1 digit after each decimal in each number, you add 1+1=2 to get the number of places you have to add back. so you get 6.25