Once we are comfortable performing operations with decimals, we can think about how to manipulate decimal quantities that we come across in the real world. Exchanging money, measuring lengths and weights, and recording times are all areas that make use of decimal numbers.
Harry buys an item from the school canteen for \$3.20. If he pays for it with a five dollar note, how much change will he get back?
How many 0.38\text{ L} bottles can be filled from a barrel which holds 41.8\text{ L}?
At midnight, the temperature in Darwin is 29.6 \degree \text{C}. Each hour after that the temperature decreases by 2.34\degree\text{C} until the sun comes up. What is the temperature 4 hours after midnight?
The solution to many real world problems will eventually involve some kind of calculation, but there is a lot we can do before and after this calculation that can make us more confident our answer is correct.
What are the quantities that we are combining?
What units do we expect the answer to have?
What operations will combine the relevant quantities to produce the expected units?
What magnitude do we expect the answer to have?
Does the answer we calculate seem appropriate in the context?