The total water in the world consists of about 1.57 x 10^20 kg of hydrogen and 1.24 x 10^21 kg of oxygen (12.4 x 10^20 kg). What is the total mass of water on earth? Remember to round approproately
$$(1.57+12.4)\times10^{20} = 13.97\times 10^{20}=1.397\times 10^{21}\:kg$$
I don't know about appropriate rounding
+4 
