Monday, October 2, 2017
Trick 3 : Group In to Twos
Same idea applies to subtraction, what's easier to think about, 15528-1210 or:
It's easier to think “28-10=18” and “55-12=43”
Carries still apply as normal:
4812 - 1598
A bit like school, 12 – 98, make it 112 – 98 = 14 carry one
1
Do 48-15 then subtract one, 48-15=33, 33 – 1 = 32
Mathematical trick
Trick 3 : Group In to Twos Same idea applies to subtraction, what's easier to think about, 15528-1210 or: It's easier to think “28-1...
Trick 2 : Slide Up or Down
You may remember from school, that subtraction was sometimes referred to as the difference between two numbers so when calculating numbers in my head, I sometimes slide BOTH numbers up or down in my head (which keeps the difference between them the same):
52
-19
I think, if I slide the 19 up one to 20, then slide the 52 up one to 53, it becomes:
53
- 20
33
This is easier to do in your head (no carries to think about)
Another example 84 - 68:
84
- 68
I think, if I slide the 84 up two to 86, then slide the 68 up two to 70, it becomes:
86
- 70
16
Again, no carries to think about. Prove this to yourself by calculating 84 - 68 = 16
Mathematical trick
Trick 2 : Slide Up or Down You may remember from school, that subtraction was sometimes referred to as the difference between two numbers so...
Trick 1 : Subtract in Bits
There's no need to subtract things all at once in your head. If you have to subtract 150 from something, you have 100 and 50 to subtract separately.
1100 - 150 → First of all, subtract the 100 to make it easier:
1100 - 150 = 1000
now all you have to do is subtract the remaining 50:
1000 - 50 = 950
Another example:
1450 - 125
I can either subtract the 100, the 20 and the 5 separately, but looking at it, I have decided to subtract the 100 and the 25 instead:
1450 - 100 = 1350
13 50 - 25 = 13 25 (I split them up in to pairs so I can think about them more easily, as explained above.
Just to explain last bit, I thought in my head:
13 50
- 25
25
This method is like the 'sliding rule' above, only thought about in a different way.
Mathematical trick
Trick 1 : Subtract in Bits There's no need to subtract things all at once in your head. If you have to subtract 150 from something, you ...
Trick 1: Addtion Multi Digit No
Add 14+12
Break into Tens and Units: 10 + 4 + 10 + 2
Add the Units: 4 + 2 = 6
Now add on the Tens: 6 + 10 + 10 = 26
Mathematical trick
Trick 1: Addtion Multi Digit No Add 14+12 Break into Tens and Units: 10 + 4 + 10 + 2 Add the Units: 4 + 2 = 6 Now add on the Tens: 6 + 10 + ...