It can be presumed that the life span in years of a certain brand of lightbulbs follows the Poisson process assumptions. Suppose that the mean life span till failure is known to be 0.7 years. To do long-term study, a series of light bulbs are arranged so that when the first one fails, the next one comes on, etc.

Determine the probability that it takes at most 3.5 years before the 4th bulb fails: