lets assume that you know what a number base system is.
there are a couple of mysteries here, one of which is the base. So let’s discover it first.
since we think alien is counting 1,2,3,4, let’s equate those letters to those digits
A=1, B=2, C=3, D=4
and let’s translate the message so far
1, 2, 3, 4, E1, 11, 21,
since CE it flipped to two digits at the fifth number, we must have a base-5 system.
in an ordinary base5 system, you’d see
1, 2, 3, 4, 10, 11, 12
in other words, we’d have 1 in the first position in 10, 11, 12
instead, the alien‘s system puts the 1 on the RIGHT end of the number, and E represents 0.
ok, that’s the other mystery, and the answer is, the alien writes numbers from Right to left, rather than left to Right.
so CBA represents what we would write as 123.
You must know know how to evaluate 123 base 5 and represent it in base 10