Time for first drop to fall 5 m:
5=21×10×t2 gives t=1 s.
Since the first drop strikes when the sixth begins, there are 5 intervals: each interval T=51=0.2 s.
When drop 1 hits the ground (at t=1 s), drop 4 has been falling for t4=1−3×0.2=0.4 s.
Distance fallen by drop 4: s4=21×10×(0.4)2=0.8 m.
Height of drop 4 above ground: 5−0.8=4.2 m.