40 men took a dip in a water tank 30 m long and 20 m broad on a religious day. If the average displacement of water by a man is 4 cu.m, then the approximate rise in the water level in the tank will be .
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Divide the total volume of water displaced by the base area of the tank.
Total displaced volume is $40 \times 4 = 160$ cubic metres, spread uniformly over the tank base of $30 \times 20 = 600$ square metres. The extra height of water equals volume divided by base area: $h = \dfrac{160}{600}$ m $= \dfrac{4}{15}$ m. Multiplying by $100$ converts metres to centimetres: $\dfrac{4}{15} \times 100 \approx 26.7$ cm, which rounds to $27$ cm. \[\boxed{27 \text{ cm}}\]