The total number of different combinations of one or more letters which can be made from the letters of the word “MISSISSIPPI” is ?

M - 1
I - 4
S - 4
P - 2

If letters can be repeated in a combination: Number of combinations = 2 * 5 * 5 * 3 - 1 (Each M can either be present or absent, similarly, each I can be present or absent and so on)
= 149

If letters should not be repeated: Number of combinations = $$^4C_1 + ^4C_2 + ^4C_3 + ^4C_4 = 2^4 - 1 = 15$$