Take Carl’s $6 out of the $16 first. That leaves Alex + Billy = 5 units.

Billy1Alex1111Carl$6$16

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

  1. Alex + Billy → $16 − $6 = $10
  2. 5 units → $10
  3. 1 unit (Billy) → $10 ÷ 5 = $2

Billy has $2.

Check: Billy $2 + Alex $8 + Carl $6 = $16. ✓