4a. The probability of satisfying the clause is .5 + .5^100. The most
common mistake was to put .5 + .5^99 which disregarded the independent
event that x1 = 0 when any of the other variables is equal to 1.
4b. No one really got this wrong. You just need to flip x1 once to satisfy
the clause.
4c. The two clauses are (~x1 OR ~x2) AND (~x1 OR x2). The common mistake
was using (~x1 or x2) AND (~x1 of x3), which can still be satisfied.