Using patterns to solve subtraction
When we looked at addition problems, we were able to see some patterns. We can also look for patterns in subtraction problems.
When we know the answer to one subtraction problem, we can use that to solve other problems. By noticing which digit has changed, such as the tens, we can make a change to our total of the same amount. Do you see any pattern here?
$16-12=4$16−12=4
$26-12=14$26−12=14
$36-12=24$36−12=24
In Video 1, we'll work through a similar example to the one above. Then we'll see if some rules or patterns old true, and test them out.
A slight variation
What if we know the total but our equation (number sentence) has a missing number- can we still follow a pattern? We sure can! Take a look at how we do this in Video 2.
What if we have to regroup?
If we can't just subtract digits in a place value column, for example, the units column, we may need to regroup, or rename some digits. Once we have done this, we can still use patterns to solve subtraction number problems. In Video 3, we solve some subtraction problems, using a pattern, then think about what rules could be true or not.
Even if you have to use regrouping, or another method, to solve one problem, you can still look for patterns in further problems.
Worked Examples
QUESTION 1
We know that $14-13=1$14−13=1.
Use this to find:
$34-13=\editable{}$34−13=
$44-13=\editable{}$44−13=
$54-13=\editable{}$54−13=
$74-13=\editable{}$74−13=
Question 2
We know that $38-25=13$38−25=13.
Use this to find:
$\editable{}-25=23$−25=23
$\editable{}-25=33$−25=33
$\editable{}-25=43$−25=43
$\editable{}-25=73$−25=73
What is the pattern? Choose the correct answer:
A two digit number minus $25$25 always has the same answer.
AWhen we subtract $25$25 to a two-digit number that ends in $8$8, we get a two-digit number that ends in $3$3.
BThe tens digit never changes when we take $25$25 away from any number.
CThere is no pattern.
D
Question 3
Complete the pattern below.
$\editable{}$ $71$71 $54$54 $37$37 $\editable{}$