At a place, the solar energy incident in one second is 640 J, on an area of one square metre. Assuming that all the solar energy is absorbed by 10 kg water, incident on its surface of area 400 `cm^(2)`. How much time will it take to raise its temperature by `70^(@)C`? (specific heat capacity of water is 4.2 J `g^(-1) .^(@)C^(-1)`)