Well, to find the answer, you need to get the fractions to have a common denominator.
Multiply 1/3 by 5/5, and multiply 1/5 by 3/3.
You get 5/15 and 3/15.
Halfway between these is 4/15.
Imagine two numbers, say, 1 and 5. The number halfway between 1 and 5 is 3. You can get 3 by adding 1 and 5 and dividing the result by 2: (1 + 5)/2 = 6/2 = 3.
Imagine another two numbers, say 4 and 16.
The number halfway between 4 and 16 is 10. You can get 10 by adding 4 and 16 and dividing the result by 2.
Generalizing, the number halfway between two numbers is equal to one-half the sum of the two numbers.