Scaling up a cake recipe using a fraction wall

Speaker: I'm just getting some bits together to do some baking, because tomorrow my son Dev is having his sixth birthday party.

I've got a recipe here that I've used before and it worked out OK.

The problem is the recipe is for 16 people and, after inviting Dev's whole class at school, I've got 22 kids coming tomorrow.

So I'm going to need to scale up the recipe and make sure it's accurate, because with baking you can't just guess.

It's got to be the right proportions, so I'm going to have to do some calculations.

It's OK though, because once Dev came along I went back to college to brush up on my maths so that'd I'd have some tricks and techniques I can use to start figuring out the answer, without panicking!

Speaker: So this is my cake recipe and it's for 16 people. And I want it to serve at least 22 people, so I'm going to need to make it bigger somehow.

I'm just going to grab some paper.

So I'm just going to draw out the 16 pieces.

OK, so I can see that half of 16 is 8, and if I add 8 to the 16 that's 24, which means if I multiply all the ingredients by 1 and a half, that will give me 24 pieces.

Well, I'm going to start with the most tricky-looking amounts and for me that's the fractions.

First one of those is 6 and a half tablespoons of milk.

In my maths class I learnt this wee table thingy called a fraction wall, which makes it easier for me to work out fractions.

So I'm gonna need half as much again on top of what the recipe says, so half of 6 and a half.

Well half of 6 is 3 and half of a half is a quarter.

So 6 plus 3 equals 9 tablespoons, plus half a tablespoon plus a quarter tablespoon.

Well, the rest of these are pretty straightforward.

I just need to get measuring these out now that I know how much I need, so I'd better get started.

Speaker: So I've got my batter now, but what about the cake tins?

The tins I normally use are 20cm in diameter and 4cm deep.

But I've got more mixture now, so I'm going to need 1 and a half times the volume for this cake mix.

So the formula for calculating the volume is πr²h, where r is the radius, h is the height and π (pi) is 3.142.

So I'll round pi down to 3 and the radius is half of the diameter, so that's 10.

So squaring a number means I need to multiply it by itself, so… 3 × 10 × 10 × 4.

1200cm³. And I multiply that by 1 and a half, which gives me 1800cm³, which is the volume I need for one tin.