135 + 265
135 + 267
137 + 267
The 3 build on eachother.
I hope you immediately see that 35 + 65 =100.
If you do not, work on complements:
If you do, also make sure you work on complements.
We will revisit them later.
When you immediately see that 135+265=400 then the next 2 are easy too.
25 X 25
There is a basic trick/shortcut for this:
(2x3) | 25 = 625
25 X 35 = (30-5)(30+5) = 30^2 - 5^ 2 = 900 - 25 = 875
23 X 69 = 23 X 70 - 23 = 20 X 70 + 3 X 70 -23 =1400 + 210 - 23 = 1600 - 13 = 1587
105 X 95 = 100^2 - 5^5 =10,000 - 25
209 - 91
Revisit complements.
The complement of 91 is 9, so 209-91 immediately turns into 109+9.
22 / 7 with rest
I hope you immediately see 3 rest 1
22 /7 with decimals
If you immediately see this as 3 rest 1, then the only thing left to be done is expanding 1/7. 1/7 = 142857 repeating, do the answer is
3.142857…
235 / 123
This is more difficult to do quickly.
123 X 2 = 246 =235 +11
So 235 / 123 = 2 rest -11
I have written a lot about division with negative remainders. Let me know if your went to know more about this.