Between 4am and 5am, the water rises from 1.4m to 1.9m. Therefore, it rises 50 centimetres in 60 minutes. This means that the water rises approximately 0.83 centimetres every minute (50 ÷ 60 = 0.83cm). The water will have to increase 0.31m – or 31cm – from 1.4m for the water to be at 1.71m (1.71 – 1.4 = 0.31m). The water will be level at 1.71m at approximately 4:38am (31 ÷ 0.83 = 37.3493976 ≈ 38)
Is this correct?
That's what I get.
x/31 = 60/50
x = 37.2
Looks closer to 37 than 38, even using your rounded-off figures.