We can double the chickens (which doubles the eggs):
3 hens will lay 3 eggs in 1.5 days.
We can double the chickens again:
6 hens will lay 6 eggs in 1.5 days.
We can now multiply the number of days by 4 (which multiplies the number of eggs by 4).
Therefore 6 hens will lay 24 eggs in a 6 days.