Pool all three amounts into one bar, then split it into 3 equal shares. Each equal share is the average.

$8$13$9pooled totalCindysisterbrother

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

  1. Total → $8 + $13 + $9 = $30
  2. Each share → $30 ÷ 3 = $10

Each will have $10.

Check: 3 × $10 = $30 = $8 + $13 + $9. ✓