Question

A DJ has a total of 1075 dance, rock, and country songs on her system. The dance selection is three times the size of the rock selection. The country selection has 105 more songs than the rock selection. How many songs on the system are dance? rock? country?

All I need are the three equations.

Answer 1

D + R + C = 1075
D = 3R
C = R + 105

Answer 2

Rock:
r
Country:
c=150+r
Dance:
d=3r

To get rock or the equation for rock you do 
150+r+3r+r= 1075

then combine like terms and use the inverse operations to get r isolated