hey buddy here is your answer!
Volume of a cuboid = l × b × h
Volume of earth dug out = 10 × 8 × 3 = 240 m^3
Volume of earh dug = 40 × 30 × height of the rectangular plot
240 = 40 × 30 × h
240 = 1200 × h
240 = 1200h
h = 240 / 1200 = 20 / 100 = 1 / 5 = 0.2
h = 0.2 m
So the plot height increased by 0.2 m or 200cm.