3.4 × 1.2
You buy three-point-four meters of ribbon at one dollar twenty per meter. How much do you pay in all?
First, estimate. Three times one is three — so the answer should land near three dollars.
Here's the trick: pretend the decimals aren't there. Multiply thirty-four times twelve as whole numbers.
Start in the ones place. Two times thirty-four is sixty-eight. Write it down.
Move to the tens place. The one really means ten, so we put a zero placeholder, then one times thirty-four is thirty-four. That gives three hundred forty.
Add the partial products. Sixty-eight plus three hundred forty is four hundred eight.
Now place the decimal. Count the digits after the decimal in each factor: one in three-point-four, and one in one-point-two. Two decimal places total.
So move the decimal two places to the left in four-oh-eight. Four hundred eight becomes four-point-zero-eight.
Check against our estimate. Four-point-zero-eight is close to three — the decimal landed in the right spot.
Three-point-four times one-point-two equals four-point-zero-eight. The ribbon costs four dollars and eight cents. Nicely done!