I'm smarter than he is:
23*54
split into := 2|3 * 5|4
2*5 = 10, 3*4 = 12, 3*5 = 15 + 1 = 16
When multiplying the middle part you need to always add 1 on the result.
Now I have:
10, 16, 12
L|R, L|R, L|R
1|0, 1|6, 1|2
Multiply out the first set by the last.
1*1 = 1
1*2 = 2
0*1 = 0
0*2 = 0
I have my first 2 digits of the answer: 12
Do the same but to middle, but leave the 1 out:
6*1 = 6 -take the 6, multiply out to the left
6*0 = 0 -take the 6, multiply out to the left
6*1 = 6 -take the 6, multiply out to the right
6*2 = 12 -take the 6, multiply out to the right
Add the result: 6+6+12 = 24, swap the digits = 42
Add the digits 12 and 42:
1242.
So what's the answer to 23*15?
23*54 = 1242
What did I get? 1242.