Well 7 x 2 = 14, 14-1 = 7, and 7+13 = 20 making the answers true.
I figured out this number by eliminating numbers, I started with 5, ended with 8 to make a range of the possible numbers. Since 5 was too low and 8 was too high I knew the number had to be either 6 or 7, so I did 7 and I got the answer.
