When you use a calculator, you must have a rough idea of the expected size of your answer.
This size is called the order of magnitude and you can get a good idea by rounding off the question first.
7840 × 28 is approximately 8000 × 30 which is 240000
24216  18947 is approximately 24000  19000 = 5000
542 ÷ 18 is approximately 500 ÷ 20 or 25
Example
Let's find the order of magnitude for 3.42 × 19.
3.42 × 19 lies between 

and 


which equal 

and 

3.42 × 19 lies between 3 × 19 and 4 × 19.
3.42 × 19 lies between 

and 


which equal 

and 

Check to see how close you are by doing the calculation on your calculator as well.
3.42 × 19 = 
