# 9.5.4 How Many Minutes for Chatting?

## Activity: How Many Minutes for Chatting?

Recall the formula for the monthly cost in $of a mobile phone that we used earlier: The owner wishes to stick to a monthly budget of$45. Starting with the “total number of minutes,” draw a doing diagram to show the operations to find the “monthly cost.” Put the monthly cost equal to $45 and then draw an undoing diagram to work out how many minutes of phone calls can be made if the monthly cost is$45.

Here is the doing diagram.

Undoing each step in turn gives the following undoing diagram.

Applying these operations in turn gives:

• Subtracting 25 gives: .

• Dividing by 0.2 gives: .

So, 130 minutes can be used for a monthly budget of \$45.

With practice, this process of using a doing-undoing diagram becomes second nature. In some cases, it can be useful if you need a formula in a different form. However, for complicated formulas, a different approach (which you will need if you continue your mathematical studies) is often used.

9.5.3 Changing Formulas

9.6 Proportion