Will the clues left behind on the question paper, put Evans back in prison again?

After using the filter paper to filter water what do you find ? What do you see left behind on the paper ? What is obtained in the beaker?

The insoluble solid left behind on the filter paper during filtration is called _________

To open a lock, a key is taken out from a collection of n keys at random. If the lock is not opend with this key, it is put back into the collection and another key is tried. The process is repeated again and again. If it is given that with only one key in the collection, the lock can be opend, then the probability that the lock will open in n trials, is