Write two numbers on the board and ask children to copy them in their notebook. Count the digits after the decimal mark of each number. For example, 23.4 has one, and 1.23 has two. Add the number of the decimal digits (1+2=3) and write the sum in a parenthesis next to the operation. Erase the decimal marks and ask children to do the multiplication. When they are all done, instruct them to start counting from the right end of the product towards the left and place the decimal mark after the third digit.
Prepare 20 flashcards containing the factors of multiplications with decimals. Each flashcard must have only one multiplication problem with big, clear digits and decimal marks. Stand in front of one student at a time and ask him to give you only the amount of numbers after the decimal mark the product must have. For example, the answer in "23.4---1.234" is four. Repeat the process with every student in the classroom.
Form a tournament consisting of one-game knockout rounds. Write down on the board five multiplication problems with decimals, including their product, but without placing the decimal marks in the product. Two students at a time come forward to compete against each other, trying to place the decimal marks on the multiplications' products as quickly as they can. Ask students not to look at the board until the problems are complete.
This activity teaches students the fast way to do multiplications with positive numbers below 1 (for example 0.3, 0.8, 0.43). Write down two random numbers, such as 0.2 and 0.4, and ask children to copy them in their notebooks. Write "0." on the product area, count aloud the digits after the decimal mark (in this case, two) and ask children to do the 2---4 multiplication. Add the result next to "0." to form 0.8. However, you need to have two digits after the product's decimal mark, so add a zero right after the decimal mark to form 0.08, the result of 0.2---0.4.