The number of ways can 5 letter be put into 3 letter boxes A, B, C if letter box A must contain atleast 2 letters ________.
Please tell what are the extra cases in this:
Choose 2 letters and put in box A – $^5C_2$
Now remaining 3 letters have $3^3$ choices
So, $^5C_2 \times 3^3 = 270$