A flock of ducks, if two are put in a cage, it's over; If three are packed in a cage, it's just over; If five are packed in a cage, it's just over;

So ducks must be multiples of 2, 3 and 5.

Least common multiple of 2,3,5 =2*3*5=30.

So the number of ducks must be a multiple of 30. Since it is more than 100, let's see how many times 30 is just more than 100, three times less than 100 and four times more than 100. So of course it is at least 120.

See at most 30* 13=390, 30* 14=420.

So 390 at most