when thinking of mode think mode most often
so the easiest why to see which is the most often
occurring number you can put the numbers in order smallest to largest
21,21,21,24,24,25,26,27,27,28,29
now go though the list and see which number showed the most
in this case 21 showed 3 times 24 showed 2 times and 27 showed 2 times
so 21 was shown the most times therefor
21 is the answer