Three times the teacher’s pay equals seven times the driver’s, so Teacher : Driver = 7 : 3. The 4-unit gap is the $2100 the teacher earns more.

Driver111Teacher1111111Teacher − Driver = 4 units = $2100

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

  1. 3 × Teacher = 7 × Driver ⇒ Teacher : Driver = 7 : 3
  2. Difference → 7 units − 3 units = 4 units → $2100
  3. 1 unit → $2100 ÷ 4 = $525
  4. Driver → 3 units = 3 × $525 = $1575

The bus driver’s monthly salary is $1575.

Check: Teacher 7 × $525 = $3675; 3 × $3675 = $11025 = 7 × $1575. ✓