Here's an example of how my head works. 39 X 13 = 507, my wife could never do that in her head.
I think 39 X10 =390, and store it in memory
I think of what's left, retaining the 390
I round the 39 to 40 , multiply by 3 to obtain 120
I add 390 +120 to get 510, but remember to subtract 3 for 390+ 120-3=507, it works for me ! I'm not calling myself brilliant, or anything ! It's the way I think.
Wife cant figure out which price is better on diet pepsi. My mind rounds it off, takes price divides it by number of cans, and comes close enough ( within a cent per can). 4-12 packs for $10 beats 3 12-packs for $8. It takes me about 1 second to see that the 3 12-packs are equivalent to $7.50. at 4/10.00. Then she asks me every time which price is better. I realize not everyone is wired the same.
The disturbing fact is she couldn't figure it out with a calculator. She doesn't really get unit pricing either !