x+3x+ 1=2 1

4x=20

x=5

So the number A is 3*5+ 1= 16.

② let a be x and b be y.

According to the meaning of the question, x+y=2 1

3y+ 1=x

(3) Substitute y=5 and x= 16 into the binary linear equations x+y=2 1.

3y+ 1=x

16+5=2 1

3*5+ 1= 16

So the original formula holds.