It depends on your technique. If you learned multiplication, #11 would be 9.75 dollars times 2 which equals 19.50 dollars. As an estimate you can round 9.75 to 8. 8x2=16 it's quite reasonable as a guess.
I am not quite sure about #12 I'll find out. I spent some time trying, but I still want to see rather what grade you are in to determine what will be best for you. I might be too under average or I might be higher than your grade