In a two digit number, the digit at the unit's place is 1 less than twice the digit at the ten's place. If the digits at unit's and ten's place are interchanged, the difference between the new and the original number is less than the original number by 20. The original number is :

Let the ten's digits be x
Unit digit be 2x - 1
Original number :
= 10x + 2x - 1
= 12x - 1
New number :
= 10(2x - 1) + x
= 20x - 10 + x
= 21x - 10
According to the question,
(21x - 10) - (12x - 1) = (12x - 1) - 20
9x - 9 = 12x - 21
3x = 12
x = 4
Original number :
= 12x - 1
= 12 × 4 - 1
= 47