Maybe she'll just try prime numbers. It's something, at least.
2, 3, 5: YES
It seems to take Keltham a while to answer this time. Maybe it really is primes, somehow?
23, 19, 41: YES
And it's faster this time. Her primes have betrayed her! Maybe the numbers just need to be pretty small somehow, in some confusing way.
2, 3, 6: YES
But it's slow again! Somehow she has something. Maybe.
Sense Motive check, is Keltham:
A) just messing with her, or
B) legitimately having trouble computing this quickly, or
C) is he wondering why she's so stupid?
Also, Willa's trying to very subtly look around the room at this point to see if everyone else is finishing and she's alone in her pathetic stupidity.