(1) Solution: If the unit number is X, then the decimal number is (2x+ 1).

(2x+ 1)× 10+x-36=x times 10+(2x+ 1)

The solution is x=3.

So the original number was 73.

(2) Solution: If the unit number is X, then the ten digits are (x- 1).

(x- 1)+x =[(x- 1)× 10+x]× 1/5

The solution is x=5.

So this two-digit number is 45.

(3) Solution: Let ten digits be x, and the unit digits are (2x).

(2x+1) ×10+(x-1) = [x times 10+2x]×2.

The solution is x=3.

So the original two-digit number is 36.

I hope I can help you, but I don't know how to ask questions. I wish you progress in your study and hope to adopt it.