I am having a difficult time solving this. Say that electrons are emitted from a source S at a very slow rate. If both slits S1 and S2 are observed, we would have roughly 50% probability of detecting an electron at one of the two slits. The interference pattern is lost and the intensity distribution will appear as the sum of two individual sources: I = I1 + I2.
But what if only one slit (S1) is observed? The observed slit (S1) will appear to produce a normal distribution, but what about the unobserved slit? This experiment has been performed with individual electrons, so we know that if both S1 and S2 are unobserved the intensity distribution contains an oscillating term for each electron. Does concluding that an electron must have passed through the unobserved slit count as an observation, and therefore destroy the interference pattern?
Edit: changed the source to electrons