Average age of 9 men is increased by 4 years when two of them whose age are 28 and 36 years are replaced by 2 persons. What is the average age of the 2 new persons ?

50 years 

Per head increase = 4 years

Total increase = 9 × 4 = 36 years

this means the two new men ,who have replaced the other two old men , are carrying 36 years  more than the sum of the ages of the men with ages 28 and 36 years i.e. ( 28+36 = 64 )

So the Sum of the ages of the two new men = 64 + 36 = 100

Therefore Average age  of two new men = \(\frac{100}{2}\) = 50 years