## Task B-2.4.

Upon arriving home, Professor Matko realized he had left his umbrella at one of the four places he visited that day: the bank, the post office, the pharmacy, and the store. The pharmacist noticed the umbrella in the pharmacy and, knowing the professor, realized that he would immediately set out to look for his umbrella. He wondered in how many different ways the professor could visit these four places, assuming that he would immediately return home after finding the umbrella. What number of visits is this about?
## Solution.

Since the umbrella is in the pharmacy, the professor could have visited one, two, or three more places before going to the pharmacy.

The number of visits to one place is 3: PBT (Post Office, Bank, Store). 1 point

The number of visits to two places is 6: PB, BP, PT, TP, BT, TB.

The number of visits to three places is also 6: PBT, PTB, TBP, TPB, BTP, BPT.

Therefore, the total number of visits that end at the pharmacy is $1+3+6+6=16$.