A rectangular cistern with base 4m by 5m has water to a depth of 6m in it. A solid metal cube with sides 3m is placed into the water. To what level in m does this raise the water in the cistern

Volume of the cube = 3 x 3 x 3 = 27m^3

Volume of the cistern = 4 X 5 x 6 = 120m^3

Total volume of the cistern and added cube = 147m^3

So.....to find the water height, h, with the cube added, we can solve this

4 * 5 * h = 147

20 * h = 147 divide both sides by 20

h = 7.35 m