well, 5/10 gives us the decimal of 5÷10 or 0.5.
8/100 gives us the decimal of 8÷100 or 0.08.
5/10 + 8/100 = 0.5 + 0.08, or 0.50 + 0.08, so they're both at the same decimal location, so 50 + 08 is 58, so 0.50 + 0.08 is 0.58.
300 + 70 = 370
370 + 0.5 + 0.8 is
370 + 0.58 = 370.58.