Subtraction By Addition
Another way of avoiding “borrowing” in subtraction is to modify the figures involved by adding the same amount to both the minuend and the subtrahend. This technique, which is also discussed in Chapter 3 of our book 25 Math Short Cuts, is very useful when the subtrahend is just below a power or a multiple of 10.
It is very similar to the subtraction by steps method we discussed in last week’s issue of our MATH-Inic Newsletter. The difference is only in the order the operations are executed.
For comparison, we will use the same examples we used in last week. Consider the amount remaining from P435 after purchasing P397 worth of goods: if we add P3 to both amounts, we will have (P435 + 3) – (P397 + 3) or an easier P438 – 400 = P38 subtraction.
Similarly, if we want to know the difference between 4321 and 3877, we can add 123 to both figures to make an easy 4444 – 4000 = 444 subtraction. Of course, as in the previously discussed technique, this subtraction by addition requires mastery of the getting the 10’s complements of numbers using Nikhilam.
Practice using subtraction by steps in the following:
- 163 – 128 =
- 134 – 89 =
- 216 – 77 =
- 544 – 268 =
- 1452 – 967 =
- 3241 – 1789 =
- 54,123 – 27,886 =
- 83,623 – 64,765 =
- 444,555 – 255,666 =
- 724,313 – 535,997 =
Answers to last week’s exercises:
- 513 – 487 = 26
- 726 – 668 = 58
- 734 – 277 = 57
- 842 – 594 = 248
- 2222 – 1879 = 343
- 3124 – 1666 =1458
- 5432 – 3778 = 1654
- 20204 – 19678 = 526
- 23232 – 18598 = 4634
- 747353 – 698765 = 48588
