How To Use The Bar Model To Solve Complex KS2 Additive Problems

Hi, I'm Zoe. And in this video we're going to be looking at one of the more complex comparison problems that our Key Stage 2 pupils are going to have to solve. In this, we're going to be using some of the techniques that we saw in our previous additive bar model problems. So if you haven't watched those videos, I suggest you stop this now and have a look at those so you're fully up to speed. This is our problem. Ahmed is 9 cm taller than Zara. Jack is 5 cm shorter than Ahmed. The total of the three heights is 406 cm. How tall is each child? Okay, we're going to use our bar model. We're going to have three. Because we have three children, we're going to start. We have to decide who is our base height. Now we can see if we have Zara, we can work out Ahmed. So Zara is going to be our base. So, so Zara is our first bar and she is an unknown number of centimeters tall. Ahmed is going to be our next bar. He's an unknown value of centimetres tall, the same as Sara, but he is 9 centimeters taller. So we can put that 9 centimeters on the end. Jack is 5 centimeters shorter than Ahmed. We can see here if we're going to take away that 5cm, it's simply the same as having 4cm taller than Zara. So we still have our unknown and that is Jack's height. We know the value of all their heights together is 406cm. So now we can solve them as we have done in our additive problems before. So 406, take away our 4 centimeters and take away our 9 centimeters gives us 393 centimeters. This represents our three unknown bars. Now, because in the previous we had two, we've now got three. Instead of dividing by two, we're going to be dividing it by three. So 393 divided by three gives us 131 centimeters. That's our unknown. We can now put this back into our bar model. So Zara is 131 centimeters. Our med is 131 plus the 9, which gives us 140 centimeters tall. Jack is 131 plus our 4 centimeters. So Jack is 135 centimeters. Now, I would encourage the children to check that this adds up to 406cm, just so they can see whether they're correct. And that's how we'd use our bar model to solve really quite complex comparison problems.