How to Teach Subtraction of Two- or Three-Digit Numbers
Subtracting two- and three-digit numbers can be difficult for young learners because it might seem counterintuitive, particularly if the student has already learned addition of two- and three-digit numbers. Situations arise in which it seems you have to subtract a larger number from a smaller number; 457 minus 298, for example, has the student subtracting 8 from 7 and 9 from 5, two seemingly impossible tasks. The key is to give the student a firm understanding of place value and how it affects the value of a number.
Instructions
-
-
1
Explain place value. The student needs to understand that a one in the second spot from the right is not actually worth one, but is worth 10; in the third spot, it is worth 100.
-
2
Perform a variety of activities, games and drills to solidify this knowledge. Teaching subtraction will be very difficult for everyone involved if the student doesn't understand the basics of place value.
For example, you can play number toss, where you write three blank spaces, a comma, and one blank space. The student gets four rolls of the dice, and can place the numbers in any of the spaces he chooses.
The first three blank spaces are the number itself, while the fourth one is the number of zeros that follow it. This will teach the child that place value can dramatically change the number's value.
-
-
3
Explain how to express a two- or three-digit subtraction problem. In this case, 457 minus 298 would be written as
457
-298
-------
-
4
Explain how numbers can be transferred laterally. So, to start the first step of the process, the 7 would become 17, and the 5 would be reduced to 4.
-
5
Tell the student to work out the new mini problem: 17 - 8 = 9
-
6
Follow the same step for the tens column. Four minus 9 is impossible, so 10 needs to be borrowed from the column to the left, turning the 4 into a 3, and the 5 into 15.
-
7
Tell the student to work out the next mini problem: 14 - 9 = 5.
-
8
Solve the final mini problem. Since this is the biggest figure, there is nothing to borrow from, but there is also no need to borrow: 3 - 2 = 1
Therefore, the answer to the problem is 457 - 298 = 159.
-
1
