### Video Transcript

15 multiplied by 6.1 equals
what?

This question involves multiplying
a two-digit number by a decimal number. Is there something we can do to one
of those numbers to make the calculation easier? Well, one strategy we could use is
to partition the number 15 into more manageable chunks. We can split 15 into 10 and
five.

We know that we can multiply
numbers by 10 quite quickly. So we can find the answer to 10
multiplied by 6.1. We can use this to find the answer
to five multiplied by 6.1 and then combine our two answers together to find the
answer to 15 multiplied by 6.1. So let’s go through these
calculations one at a time.

What is 10 multiplied by 6.1 or, if
we look at it the other way, 6.1 times 10? We know that when any number is
multiplied by 10, the digits in that number become 10 times larger. To help us understand this, let’s
draw a place value grid. 6.1 is made up of six ones and
one-tenth. When we multiply this by 10, the
number of ones in our number, six ones, are going to become 10 times as large. Six ones become six tens or 60. And we can see that that digit,
six, has shifted one place to the left.

What happens to a number of tenths
if we multiply it by 10? Well, ten-tenths are one whole. And so if we make one-tenth 10
times as large, it will become one whole. The digit once again has shifted
one place to the left. And so we know 10 times 6.1 or 6.1
times 10 equals 61.

Now we need to find five multiplied
by 6.1. We already know that 10 lots of 6.1
are 61. So how can we use this to help? Five is half of 10. And so if we halve 61, we can find
the answer to five lots of 6.1. What’s half of 61? Half of 60 is 30, and half of one
is 0.5. So half of 61 must be 30.5.

So now we have all the information
we need to be able to say what 15 multiplied by 6.1 is. Because we know 15 is made up of 10
and five, we can add 10 lots of 6.1 and five lots of 6.1 to find the overall
answer. What’s 61 plus 30.5? 91.5, which is the same as saying
91 and a half.

This is not the only way we
could’ve found the answer. Remember, at the start, we talked
about how we could break up 15 to make it more manageable. Well, we could also do the same to
6.1. We can partition 6.1 into six and
0.1. Then just like before, we can work
out the answer to both parts, 15 multiplied by six and then 15 multiplied by 0.1 or
one-tenth, and then as before combine them to find the overall answer.

What’s 15 multiplied by six? Well, there are lots of strategies
we could use. One way is to partition 15 once
again. 10 lots of six plus five lots of
six equals 15 lots of six. We know that 10 lots of six is
60. Five lots of six must be half of
this, which is 30. And the total of 60 and 30 equals
90. So 15 multiplied by six must be
90.

What’s 15 multiplied by 0.1? This isn’t as hard as it looks. 0.1 is the same as one-tenth. And so multiplying 15 by one-tenth
is really the same as saying what’s one-tenth of 15. When we find one-tenth of a number,
the digits shift one place to the right. They become 10 times less. And so one-tenth of 15 is 1.5.

So now all we need to do is to
combine the two parts back together. 90 plus 1.5 equals 91.5. Really, the way we’ve found the
answer is to use the same strategy. We started off by partitioning one
of the numbers to make it easier. Then we multiplied both parts, one
by one, using known facts to help us. Then we combined the answers to
both parts back together again to find the overall total. 15 multiplied by 6.1 equals
91.5.