3.1.2 Subtraction

Consider the following questions:

  • What’s the difference in distance between taking the Eighth Street Bridge versus MLK Boulevard?
  • How much more money do we need to save?
  • If we take away 45 of the plants for the front garden, how many will be left for the back garden?
  • If all vacation prices have been decreased (or reduced ) by $20, how much is this one?

All these questions involve the process of subtraction to find the answer—a process that you often meet when dealing with money. You can see that the “trigger” words for subtraction are again in bold.

One way you can think of subtraction is as the process that undoes addition. For example, instead of saying “$10 minus $7.85 leaves what?” you could say, “What would I have to add to $7.85 to get to $10?”

Adding on 5 cents gives $7.90, another 10 cents gives $8, and another $2 will give you a total of $10. So, the total amount to add on is equation left hand side sum with, 3 , summands dollar times 0.05 plus dollar times 0.10 plus dollar times 2.00 equals right hand side dollar times 2.15. This is the same answer as the one obtained by subtraction: equation left hand side dollar times 10 minus dollar times 7.85 equals right hand side dollar times 2.15.

In other words, you can always check or work out a subtraction problem by using addition. For example, if you calculate 1874 – 1476 = 398, then a quick check is to make sure 1476 plus 398 equals 1874, which it does.

Working out calculations like this on an everyday basis may help you to feel more confident in working with numbers. You can always check your answer on a calculator, if you like.

Subtraction in the Real World