Correct Answer - B
Given,dimensions of the plank are
`l = 4m, b = 50cm = (50)/(100)m = 0.5m`
and `" "h= 20 cm =(20)/(100) m = 0.2m`
`therefore` Volume of the plank `= l xxb xxh = 4 xx 0.5 xx 0.2 = 4m^(3)`
Also, gien dimensions of the pit are
l = 16m, b= 12m and h = 49 m
`therefore` Volume of a pit `= l xx b xxh = (16 xx12xx40)m^(3)`
`therefore` Number of planks `=("Volume of the pit ")/("Volume of the plank") = (16 xx 12 xx 40)/( 4)`
`= 16 xx 12 xx 10 = 1920`
Hence, the number of planks are 1920.