Point C is 0.55. This is because it is the centre point between 0.5 and 0.6. Adding value back into these numbers, 0.5 becomes 0.50 and 0.6 becomes 0.60. Moving aside the decimal, 55 is the midpoint between 50 and 60 and so 0.55 should be the middle point between 0.5 and 0.6.
I hope this helps :)