Let the length of flagstaff is x.
Shadow length of stick : Its length :: Shadow length of flagstaff : Its length
4 m:3 m::24 m:x
43=24x [ if, a,b,c and d are in proportion, then ab=cd]
4×x=3×24
x=3×244 [by cross multiplication]
x = 18
Hence, the flagstaff is 18 m tall.