Cross-multiplication: Solve 4/7 = 12/x

Set the two ratios equal and cross-multiply (or find the unit rate, then scale). The correct answer is 21. Traps to avoid: 15 comes from the "added terms" error; 24 comes from the "doubled c" error; 7 comes from the "ignored the scale" error; 6.90 comes from the "inverted cross multiply" error.