9.5.4 How Many Minutes for Chatting?

Activity Symbol Activity: How Many Minutes for Chatting?

Recall the formula for the monthly cost in $ of a mobile phone that we used earlier:

monthly cost in dollar equation left hand side equals right hand side 25 plus open total number of minutes negative 30 close multiplication 0.2

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.

Answer Symbol


Here is the doing diagram.

Undoing each step in turn gives the following undoing diagram.

Applying these operations in turn gives:

  • Subtracting 25 gives: 45 minus 25 equals 20.

  • Dividing by 0.2 gives: 20 division 0.2 equals 100.

  • Adding 30 gives: 100 plus 30 equals 130.

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