Fifth grade students in Ms. Bailey’s class at Davis Elementary have been learning how to multiply decimals by two-digit numbers using paper and pencil (and sometimes calculators). Today I showed them how to write formulas in a Numbers spreadsheet that will do the same thing. This activity helps them break up the process into logical steps so they can see how it all works together. First I gave them a template that you can download by clicking here. They typed in a number of their choice with a decimal (to the hundreds place) and another 2-digit number to multiply by. Next I showed them how to write the first couple of formulas to multiply the ones digit in the bottom factor by different digits in the top factor. Once they got the hang of it, they were able to complete the rest of the formulas on their own. Then we wrote formulas for multiplying the tens digit of the bottom factor by each digit in the top factor. It was a great review of place value and the distributive property! The next step was to add up all the products to get the final answer. Finally, we wrote a formula multiplying the two numbers to check if it equaled the sum of all the products of the individual digits. It really seemed to click with the students. The only problem was, when they wrote a new number, they would have to rewrite all the formulas. So afterwards I came up with another activity that you can download by clicking here. It breaks up any number they write into place value digits (that was hard to figure out!). Then they can write formulas that will work with any number.
