For 30% of the points, since Larry always sends his message after Theodore last checks his phone, we can simply output Who knows....

For the rest of the points, note that we only have to consider three relevant times. Let $M$ be the time when Larry starts his homework, and $I$ be the interval. Let $T$ be the time that Larry sent his message. We can simply check the $3$ times $M+I$, $M+2I$ and $M+3I$. If $T$ is strictly greater than all of them, then we output Who knows..., otherwise we output the earliest time.