Ah! Now I see.
I checked the archive of the puzzles and I see that 378 can be got from 18x21.
The 21 was already given so to get the 18, I divided 8 by 4 and subtracted the result from the 20.
I checked the archive of the puzzles and I see that 378 can be got from 18x21.
The 21 was already given so to get the 18, I divided 8 by 4 and subtracted the result from the 20.