The sequence doesn't appear to have any missing numbers. (Lol)

The next number in the sequence is possibly 8.

The rules seem to alternate between addition and division. As the sequence progresses, the number being added or divided by is apparently increasing by 1 each time.

40 (start)

42 = 40 + 2

21 = 42 / 2

24 = 21 + 3

8 = 24 / 3

12 = 8 + 4

3 = 12 / 4

8 = 3 + 5 (proposed next number in sequence)

1 3/5 = 8 / 5

7 3/5 = 1 3/5 + 6

1 4/15 = (7 3/5) / 6

The next number in the sequence is possibly 8.

The rules seem to alternate between addition and division. As the sequence progresses, the number being added or divided by is apparently increasing by 1 each time.

40 (start)

42 = 40 + 2

21 = 42 / 2

24 = 21 + 3

8 = 24 / 3

12 = 8 + 4

3 = 12 / 4

8 = 3 + 5 (proposed next number in sequence)

1 3/5 = 8 / 5

7 3/5 = 1 3/5 + 6

1 4/15 = (7 3/5) / 6