Mr. Butler is filling planter boxes with soil. He has soil that is 30% sand, and he buys 9 pounds of a commercial potting soil that is 18% sand. He mixes some of his soil with the 9 pounds of commercial potting soil. The resulting soil mixture is 21% sand. How many pounds of his soil did Mr. Butler use?
Let the amount of Mr. Butler's 30% sand =S
0.30S + 0.18(9) =0.21(9+S)
0.30S + 1.62 =1.89 + 0.21S
0.30S - 0.21S =1.89 - 1.62
S =0.27 / 0.09
S = 3 pounds - of 30% sand that Mr. Butler used.