Cross-multiplication: Solve 2/5 = 6/x

Set the two ratios equal and cross-multiply (or find the unit rate, then scale). The correct answer is 15. Traps to avoid: 9 comes from the "added terms" error; 12 comes from the "doubled c" error; 5 comes from the "ignored the scale" error; 2.40 comes from the "inverted cross multiply" error.