24 x 12

The huge Excel handbook had been my intense focus as a potential deskside banker support person.
 
On a third round at a big investment bank, a banker asked "What's 24 x 12?"
I said "288" He asked me how -- "12 x 12 doubled is 24 12s" = 288
He responded 24 x 10 + 24 x 2 = 240 + 48 = 288

I then said 24 x 24 = 576 -- take half of this (12 24s)which is 288

Both of us were at least 30, so I then said:
 "If you remember your high school algebra -- 24 and 12 are both 6 from 18 so you can write 24 x 12 as (18 + 6)(18-6) and when you FOIL that  it becomes 18^2 + 6(18) - 6(18) - 6^2. which is 324 - 36 = 288!"

I got the job!