Matilda is looking at making a concrete mix. She mixes 2 parts of cement powder to every 7 parts of sand. Cement costs 35 cents a kilogram and sand costs 20 cents a kilogram. In total she spent $180. What was the total weight of cement she purchased?
c/s = 2/7 so s = 7 c/2
.35 c +.2 s = 180
.35 c + .2 (7 c/2) = 180
.35 c + .7 c = 180
1.05 c = 180
c = 171 kg