How many ways can 3 letters be arranged in 4 envelopes?

Uttarakhand Power Corporation Limited (UPCL) is soon going to announce the official notification for the UPCL AE 2022. A total of 105 vacancies were released for the previous recruitment cycle and the UPCL is expected to release more vacancies for this year. Candidates with Graduation degrees are only eligible to apply for the recruitment process and after selection will get a salary range between Rs. 56,100 to Rs. 2,06,700. Candidates who want a successful selection must refer to the UPCL AE Previous Year Question Papers to boost their preparation for the exam.

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Question is : In how many ways can the letters be put into the envelopes, so that at least one letter goes to the correct address ?

Answer is 15.
All i could think of is : 4 envelop choices for each letter, so 4*4 = 16.

A little fertilizer for my withered grey-matter ?