21. What number comes next in the sequence below 15,12,24,20,33 42,. a) 44 b) 36 c) 52

To find the next number in the sequence 15, 12, 24, 20, 33, 42, let's first look for a pattern:

1. From 15 to 12: -3
2. From 12 to 24: +12
3. From 24 to 20: -4
4. From 20 to 33: +13
5. From 33 to 42: +9

Now let's observe the differences:

-3, +12, -4, +13, +9

The positive numbers seem to be increasing in a somewhat erratic manner. To further analyze, consider only observing the positive increment:

• The +12 and +13 have an increment of +1,
• Following that, if we were to continue this pattern by looking at the next positive increment, we might consider +10, giving us:

42 + 10 = 52.

So the next number in the sequence is likely 52.

Thus, the answer is: c) 52.