Transcript Problems like ? + 29 = 81

Problems like … + 29 = 81
I am learning to reverse problems like □ + 29 = 81 to 29 + □ = 81
and then use an appropriate mental method to solve the problem.
By now you know a number of
different methods. Let’s try to
choose the best method for each
type of question.
This means working it out in
your head.
First let’s check that the order (e.g. ? + 3 = 8 or 3 + ? = 8) doesn’t
matter.
Zane has some counters, he gets 3
Zane has 3 counters, he gets some
more and now has 8 counters. How
more and now has 8 counters. How
many counters did he start with?
many counters did he get?
?+3=8
3+?=8
Below are the total amount of
counters we should end up with.
Below are the total amount of
counters we should end up with.
We don’t know how many counters
Zane starts with but we know the
second amount of counters he got
was 3.
Now let’s record the three counters
he starts with.
You can see from the diagram above
Zane must have stated with 5
counters. 5 + 3 = 8
You can see from the diagram above
Zane must have been given 5
counters. 3 +5 = 8
Notice how the answer (5) is the
same using both methods.
Kirsty has some money and earns $18 more. She now has $46.
How much money did she start with? ? + 18 = 46
Step 1: Change the question around. 18 + ? = 46
Step 2: Choose an appropriate method. A good choice here would be
‘Jumping the Number Line”.
Try to do each step in your head before clicking
Step 3: Place the starting and end numbers on the number line.
Step 4: Choose a tidy number to jump to.
Step 5: Jump from the tidy number (20) up to the target number (46).
Step 6: Add the jumps together. This is your answer.
2 + 26 = 28
18 + 28 = 46
+26
+2
18
so
20
46
A farmer has some sheep. He buys 13 more. He now has 87. How
many sheep did he start with? ? + 13 = 87
Step 1: Change the question around. 13 + ? = 87
Step 2: Choose an appropriate method. A good choice here would be
adding the ones and tens separately (see “Problems like 37 + … = 79).
Try to do each step in your head before clicking
Step 3: We have 3 ones and need 7 ones. How many more ones do
we need?
4
Step 4: We have 1 ten but need 8 tens. How many more tens do we
7 tens (70)
need?
Step 5: Add these amounts together. This is how many sheep the
farmer must have started with. 70 + 4 = 74
so
13 + 74 = 87
Now try these questions on your own.
Remember to first change the question then answer.
1) … + 67 = 84
67 + 17 = 84
2) … + 19 = 56
19 + 37 = 56
3) … + 13 = 75
13 + 62 = 75
4) …. + 67 = 79
67 + 12 = 79
5) … + 21 = 82
21 + 61 = 82
Click again when you have
completed each question and
the answer will be displayed.
Need some more practise? Try Pg49 of Teacher Tools Addition and
Subtraction Book Numeracy resources