Let thickness of the bottom of the cistern be x cm; then internal measurements are 330 – 2 × 5 = 320 cm; 260 – 2 × 5 = 250 cm and 110 – x cm respectively. Since, 320 × 250 × (110 – x) = 8000 × 1000 ⇒ 110 – x = 320×2508000×1000​ ⇒ 110 – x = 100 ∴ x = 110 – 100 = 10 cm