For a program of k variables, boundary value analysis yields ______ test cases.

Explanation

Boundary value analysis varies one variable at a time while holding all the others at their nominal (typical) value. For each variable four boundary-related values are tried — minimum, just above minimum, just below maximum and maximum — which gives 4 test cases per variable, so 4k in all. One extra test case is added in which every variable simultaneously takes its nominal value. Total = 4k + 1 test cases. (Robustness testing, which also adds values just below the minimum and just above the maximum, is the variant that yields 6k + 1.)