Just draw the picture.

Construct a pushdown automata M, such that where = {a,b,c). For example, your PDA should accept the strings: aab, abbe, and aaabbbcccc, but reject the strings: abc, abbac, aabbcc. UPDATE: I originally had this question for n 20 instead of n 2 1. Both ways are doable but one has a few less states. You can choose to do it either way and be granted full grades. That is, you can choose to have your machine accept e or not. [10 Construct a pushdown automata M, such that where = {a,b,c). For example, your PDA should accept the strings: aab, abbe, and aaabbbcccc, but reject the strings: abc, abbac, aabbcc. UPDATE: I originally had this question for n 20 instead of n 2 1. Both ways are doable but one has a few less states. You can choose to do it either way and be granted full grades. That is, you can choose to have your machine accept e or not. [10