For adding, it goes up the number you add with. Subtracting,it goes down the number you add with. But multiplying, if its a decimal with a number then or no just a decimal, like 23.94 or 0.23 your number goes down.<em /> The reason is that when multiplying it goes up, but multiply by 1, its the same, so if it's less than 1, it goes down.
In bmultiplying, you always have to move the decimal of the product the amount of place values the decimal has in each factor, for example, 1.50 times 1.50= 2.25, the factors together have 4 decimals, so you move the decimal 4 times to the right (try it, it gets rid of alot of zeros in the answer.)
