Split the $600 into 10 equal parts (each = 10%). Savings are 20% = 2 parts.

Plain numbers are given; italic underlined numbers are worked out.

1. 100% → $600, so 10% → $600 ÷ 10 = $60
2. Saves 20% → 2 × $60 = $120

Wendy saves $120.

Check: 20% of $600 = 0.2 × 600 = $120. ✓