Each monster has six eyes. How many eyes do eight monsters have?
The first piece of information that we’re told in this problem is that each monster has six eyes. There are five on top of the head and one big one in the middle. The question then goes on to ask us, how many eyes do eight monsters have? And so, we need to use what we know about one monster to help us find the total eyes that eight monsters have altogether.
Because we’re finding several lots of the same amount, one way to find the answer could be to add six add six add six eight times. But there is a quicker way. Repeating the same addition lots of times is the same as multiplying. There are eight monsters and there are six eyes on each monster. So, we’re looking for the total of eight multiplied by six.
How can we find eight sixes? Perhaps you know the multiplication fact already. Another method might be to skip-counting in sixes. This is similar to adding six over and over again. So, there are quicker methods we could use. Another method we could use, because we’re multiplying by eight, is to use doubling to help. Let’s have a go at using this method.
The key fact to remember is that when a number is multiplied by eight, it’s doubled, doubled then doubled again. Let’s show how this works using our bar model. So, we need to multiply six by eight. Starting with the number six we double. Double six is 12. That’s the same as having two lots of six.
Next, we need to double 12. We double the ones to get four and double the tens to get 20, 24. We started with six. We doubled, and we doubled again. That’s the same as finding six times four. Now we need to double one more time. We can see that by doubling a third time, we’ll be able to find out the total number of eyes that all eight monsters have.
What’s double 24? Double the ones. Two lots of four is eight. And double the tens. Two lots of 20 is 40. And so, 24 doubled equals 48. So, by using our double-double-double method we were able to multiply six by eight. And so, as we look at this row of monsters we can see 48 eyes staring back at us.