how to find the middle of 42.3 and 47.9?

If you're looking to find the "average" of the two numbers, you simply add the two numbers together and then divide that number by 2.
42.3 + 47.9 = 90.2
90.2 / 2 = 45.1

(42.3+47.9)/2=45.1

(42.3 + 47.9)/2
= 90.2/2
= 45.1