What an intriguing question!
Note that the first 10 multiples of 3 end on every possible digit from 0 to 9.
3," "6," "9," "12," "15," "18," "21," "24," "27," "303, 6, 9, 12, 15, 18, 21, 24, 27, 30
Arranged so the units are in order from 00 to 9:9:
30," "21," "12," "3," "24," "15," "6," "27," "18," "930, 21, 12, 3, 24, 15, 6, 27, 18, 9
Every number from 2020 upwards can be written either as a multiple of 3 or 103or10, or as the sum of a multiple of 1010 and a multiple of33.
20 = 10+1020=10+10
21 = 7 xx321=7×3
22 = 10+12 = 10 +4 xx322=10+12=10+4×3
23 = 10+323=10+3
25 = 10+15 = 10+5xx325=10+15=10+5×3
26 = 20+2xx326=20+2×3
28 = 10 +18 = 10 +6xx328=10+18=10+6×3
29 = 10+3xx329=10+3×3
31 = 10 +21 = 10 +7xx331=10+21=10+7×3
32 = 20+12 = 20+4xx332=20+12=20+4×3 etc.....
What about the numbers less than 20 that cannot be formed with 3 and 10?3and10?
19 = 10+9 = 10+3xx319=10+9=10+3×3
18 = 6xx318=6×3
17 = ???????????????17=???????????????
