Let the numerator = x
and the denominator = y
So, the fraction = x/y
According to the question,
Condition I:
(x + 1)/(y - 1) = 1
⇒ x + 1 = y – 1
⇒ x – y = – 2
⇒ x – y + 2 = 0 …(i)
Condition II:
x/(y + 1) = 12
⇒ 2x = y + 1
⇒ 2x – y = 1
⇒ 2x – y – 1 = 0 …(ii)
By cross - multiplication method, we have

On taking I and III ratio, we get
⇒ x/3 = 1/1
⇒ x = 3
On taking II and III ratio, we get
⇒ y/5 = 1/1
⇒ y = 5
So, the numerator is 3 and the denominator is 5
Hence, the fraction is 3/5.