Two similar television sets and one DVD player cost $919.70. One television set and two similar DVD players cost $639.70. How much do three television sets cost?
2t+d = 919.70
t+2d = 639.70
substituting,
t + 2(919.70-2t) = 639.70
-3t + 1839.40 = 639,70
3t = 1199.70
or, you could subtract to get
t-d = 280
2t+d = 919.70
now add to get
3t = 1199.70