How To Multiply Decimals
5.7 x 3.9 may appear ___________ at the beginning.
Hi,my name's Charles and I'm one of the math teachers from the Maxim Workshop. I'm just going to now teach you how to do some math. I'm going to show you how to multiply decimals. Now, decimals are slightly trickier than multiplying integers because there's a little bit of manipulation you need with the decimal point at the beginning. So imagine you're given a particular question like 7.2 times 4. The first thing you want to do is move the decimal place to the right. That is the equivalent of saying 7.2 times 4 times 10. This, when multiplied by this gives you 72 times 4. Now it's important to realize that you added this extra factor here so in the slot, you need to remind yourself to divide the one slot by ten. That will simply be to move the decimal point to the left at the end. So, now that we have this, this looks a lot easier to multiply by ten. Just to take our decimal point past our number. I'm going to write this so we'll remember this 7.2 times 4. That is our question, but we are multiplying 72 times 4. Now again, we do four times two, which is eight, Four times seven, which is twenty-eight. So we have arrived at 288. Remember we are trying to figure out what 7.2 times 4 is and we only multiplied this by ten which has the affect of basically moving the decimal point one jump to the right. For our answer, we need to move the decimal point, which is here, one place over to the left. As this stands, this is equal, so you need to go over here and say 7.2 times 4 is equal to 28.8. Now this is the answer to this, now what we're going to try to do is work with two numbers that have decimals in them and multiply together. The question that I'm going to throw out now is what is 5.7 times 3.9? Now, this does look tricky at the beginning but if you just use the technique of bringing back the decimal point each number, you how many places to move the decimal place forward on the answer. The question we have here is 57, 39. This all arrives from multiplying this by ten and this by ten. That has an effect of really just bringing this decimal point here, and this decimal point here, which is two times. You might want to leave a marker at the side of your page saying decimal point times two. This will remind you to bring the decimal point forward twice. Now, we have to figure out what this comes to. We have nine times seven, which is sixty three so you put the three down there, carry the six. Nine times five which is forty five. Once you carry the the six, you're going to get 51 so put that down here. Now if you've got your next column here, you've got the number three which is going to multiply by the seven there. Remember we always start off one number to the left for these columns for the tens unit. If it was the hundreds ten, we'd start off three columns but as we're starting off with the three, start to the left for this column. Seven times three is twenty one. Put the one there,put the two there as we're carrying 21 and three times five is fifteen add the two makes seventeen. We have three plus zero which is zero. One plus one which is two. Five plus seven which is 12, carry the one. One plus one is two again. Now, remember this is the marker that tells us the we need to bring decimal points forward twice. We have two thousand two hundred and twenty so what we need to do is say 5.7 times 3.9 is equated to, one, two, so that's 22.20 or 22.2. That's basically how to multiply decimals.