I'm really stuck on this question and if anyone could help me with it I'd be so grateful

Answer:
150
Step-by-step explanation:
Rate at which volume decrease:
(1 × 2 × 20/100) ÷ 30
= 1/75 m³/min
Total volume:
½ × (1.4 + 0.6) × 2 × 1
= 2 m³
Time = 2 ÷ 1/75
= 150 mins