In order to round, I like to memorize where the places in a decimal are first. Then you find what you need to round, in your instance the nearest hundreth. The nearest hundreth is 0 . The rule I have learned and there's a song which goes like...
Find your place Look next door Five or greater, add 1 more All digits in front stay the same, All digits behind, zero's your name!
Since it's a 0 in the hundredths place and its less than 5, it stays 0 and doesn't add up. Therefore you can tell that 15.5 is already rounded to the nearest tenths