Rates from Unequal Watching Times

Events occur as a Poisson process with a constant unknown rate λ per hour. The process is watched during three separate, non-overlapping windows. In a 2-hour window 3 events are seen, in a 5-hour window 10 events are seen, and in a 1-hour window 0 events are seen. What is the maximum likelihood estimate of λ, in events per hour? Give an exact fraction.