Discussion

Explanation:

Here letters are similar, but the letter boxes are distinct.

Let the three distinct letter boxes get x, y and z letters.

∴ x + y + z = 4.

Note: This is same as distributing n identical item to r different people. 

∴ The required answer = ^4+3-1C_3-1 = ⁶C₂ = 15 ways.

Hence, 15.

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