Explanation:

Average of a and b = ( a + b ) /2 --( 1 )
16a + 16b = 48 [ given ]
Divide each term with 16 , we get
a + b = 3
( a + b ) / 2 = 3/2
Therefore , average of a and b 
= ( a + b ) /2 = 3/2 = 1.5

Hence, option 3 is the correct answer.