Let the original number be 10p + q.
So from question,
q = 2p + 1 ...(i)
and (10q + p) - (10p + q ) = (10p + q) - 1
⇒ 9q - 9p = 10p + q -1
⇒ 19p - 8q = 1...(ii)
Putting the value of (i) in equation (ii) we get
19p - 8(2p + 1) = 1
⇒ 19p - 16p - 8 = 1
⇒ 3p = 9
⇒ p = 3
So, q = 2 x 3 + 1 = 7
∴ Original number = 10 x 3 + 7 = 37