Algebra: A Combined Approach (4th Edition)

Let $x$ be the volume of 40% acid solution needed. $(30-x)$ is the volume of 10% acid solution needed. 40% = 0.4; 10% = 0.1; 20 = 0.2 $0.4 \times x + 0.1 \times (30-x)=0.2 \times 30$ $0.4x + 3-0.1x=6$ $0.3x+3=6$ $0.3x=3$ $x=10$ 10 gallons of 40% acid solution needed. $30-x=30-10=20$ 20 gallons of 10% acid solution needed.