Even Letters
How many strings of length 10 over the alphabet {A, B, C, D} contain an even number of A's and an even number of B's? (Zero counts as even.) Give an integer.
Related problems & prerequisites
Worth solving first
- Fibonacci in Decimal 5.0/10
Source: Classic exponential-generating-function exercise in wide circulation; no traceable original source. Statement written for AxiomIQ.