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.
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.