Unitary method

hint: score = 20

Explain

12 pencils cost $21
20 pencils cost $X

solve for X ...

Unitary method, RP.

12 cost $21, 1 costs ?..

never heard of it, but that's not the question.

knowing how much 1 costs will certainly help in deriving the correct answer...

The unitary method is pretty useless if the total does not divide "nicely".
Better to use $x:20 as $21:12

NB I note that both of the questions here are equivalent to UK 11 year old level. I'm just wondering how old the questioner is, whose homework we are doing.

If a pencil costs 21 dollars, the cost is gonna be some sort of a revolution and bloodshed.

You are having reading issues, aren't you?