Video Transcript
The distance between two cities is 18523 kilometres. What distance would a car need to cover to make one round trip from one city to the other and then back again?
This problem is all about calculating the distance of a journey. The first piece of information we’re given is that the distance between two cities is 18523 kilometres. And the journey that we need to calculate is a round trip that a car makes from one city to the other and then back again. In other words, the car needs to travel 18523 kilometres in one direction and then exactly the same distance back again.
We can find the total distance by adding two lots of 18523. But perhaps a quicker way to find the answer is to multiply this number by two. And we can do this using short multiplication. First, we need to write out the number that we’re multiplying by two and then the number that we’re multiplying by, which is two, underneath. And the aim is to go through each of our five digits, cause this is a five-digit number, and multiply each one by two, starting with the ones digit.
Three ones multiplied by two equals six ones. In the tens place, we have the digit two. Two 10s multiplied by two equals four 10s. Of course, by multiplying each of these digits by two, what we’re doing is doubling them. Onto the hundreds, five 100s doubled. We know 500 times two is one 10 100s, or 1000. So, we exchange 10 100s for 1000.
Eight 1000s doubled equals 16 1000s. Remember, we’ve already exchanged 1000, so that’s 17 1000s. And we can write this as seven 1000s and one lot of 10000. Finally, one lot of 10000 multiplied by two equals two lots of 10000, or 20000. We’ve already got one lot of 10000 underneath, so that’s 30000 altogether.
If the distance between two cities is 18523 kilometres, then the distance a car would need to cover to make a round trip from one city to the other and then back again would be 37046 kilometres. We found the answer using short multiplication.
