In this activity, we will solve real life problems by substituting values with two unknowns to evaluate a formula.

Remember - To substitute just means to swap something.

Like with football - one player comes off the pitch and one comes on in their place!

They are swapped!

In maths, it looks like this with an expression:

When **a = 5 **and **b = 10 **

What is the value of **2a + 4b**?

**Answer**

2a + 4b = (2 x a) + (4 x b)

When a = 5 and b = 10 we substitute them

(2 x 5) + (4 x 10)

= 10 + 40

= **50**

Let's look at a real life formula problem!

**Example**

The cost of hiring an electric bike is found using the following:

**Cost = £4 x number of hours hired + £5 for bike lock**

How much would it cost to hire a bike for 3 hours?

**Answer**

We substitute what we know into the problem.

We know that the bike is hired for 3 hours.

Cost = £4 x number of hours hired + £5 for bike lock

Cost = £4 x 3 hours + £5

Cost = 12 + 5

**Cost = £17 for 3 hours hire**

Let's try some more like this!