Answer: 1.2 x 10^7kg
Explanation:
Volume of tank = length x width x height
Volume = 51 x 20 x 11
Volume = 11220 m^3
Density = Mass/Volume
Density = 1030 kg/m^3
Volume = 11220 m^3
Mass = m
1030 = m/11220
m = 1030 x 11220
m = 11,556,600 kg or 1.2 x 10^7kg