Explanation:
A "100 chart" is a listing of all the numbers 1 to 100, in rows of 10. This results in columns of numbers that all end in the same digit. For your purpose of instructing your little brother, I recommend you web-search for "100 chart" and print one of the ones available.
Locating the given numbers on a "100 chart", you find they are in the last 4 columns of the rows ending in 20, 30, 40, 50. Overlaying the boxes onto the chart, you see they correspond to number positions ...
17 _ _ 20
27 _ 29 _
_ 38 _ 40
47 _ _ 50
