Explanation:

If M₁ persons can do W₁ works in D₁ days working T₁ hours a day and M₂ persons can do W₂ works in D₂ days working T₂ hours a day then we have a short-trick formula which is M₁D₁T₁W₂ = M₂D₂T₂W₁
Given:
M₁ = 30, D₁ = 24 days, W₁ = 1500 units, T₁ = 6 hours a day
M₂ = 18, D₂ = ?, W₂ = 1800 units, T₁ = 8 hours a day
By the short trick apporach,
M₁D₁T₁W₂ = M₂D₂T₂W₁
⇒ 30 × 24 × 6 × 1800 = 18 × D₂ × 8 × 1500
⇒ D₂ = 30 × 24 × 6 × 1800 /18 × 8 × 1500
⇒  D₂ = 36 days