If the last number is 1 -> 21 <50
if the last number is 3 -> 63 > 50 OK
if the last number is 5 -> 105 which is not a two digits number
if the first number is 4 -> 42<50
if the first number is 6 -> we get 63 again
if the first number is 8 -> we get 83 which is not odd
63 is the only answer
