Don't worry about the decimals. There's an easy trick at the end.

I assume you can do the long multiplication?

Ignore the decimal points and multiply 655 by 312=204360. Now to insert the decimal point.

Count the number of decimal places for each number. There are two for each. Add the 2's together to get 4, now count 4 places from the right in 204360 and put in the dec pt: 20.4360.

Another way to work out where the dec pt goes is to use estimates.

Look at the two numbers and multiply their integer (whole number) parts: 6*3=18. The answer is going to be close to 18.

Use the Goldilocks method: you can't put the dec pt just after the 2, the answer would be too small; you can't put it after 204, it's too big; but after 20, it's just right!