How many four letter codes can be formed using letters A C

How many four - letter codes can be formed using letters A, C, E, T, S, H if (a) repeated letters are allowed (b) letters can not be be repeated

Solution

there are 6 letters

a) If repeated then each space of 4 letter code can be filled by every given letter

So, no. of codes possible= 6*6*6*6 = 6^4 =1296

b) If letters cannot be repated then f1st space can be filledby 6 ways, 2nd space by 5 ways , 3rd space by 4 ways and 4rth space can be filled by 3 ways

So, no. of codes possible = 6*5*4*3 = 360 ways