To find the greatest decimal, compare place values left to right: ones, tenths, hundredths, thousandths.

Numbers	=	3.115, 3.511, 3.500, 3.150
All have ones digit 3. Compare tenths: 1, 5, 5, 1. 5 is greater. Between 3.511 and 3.500, compare hundredths: 1 > 0. So 3.511 is the greatest.
Greatest	=	3.511