5) (60a-30): 5 = 18

Решение:

1. \[ (60a - 30) : 5 = 18 \]
2. \[ 60a - 30 = 18 \times 5 \]
3. \[ 60a - 30 = 90 \]
4. \[ 60a = 90 + 30 \]
5. \[ 60a = 120 \]
6. \[ a = 120 : 60 \]
7. \[ a = 2 \]

Ответ: a = 2