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**melody** (a) How many strings of length 3 can be formed from the alphabet {'0' ,....., '9'} if each string must include '5' or '6'?

(b) We would like to use our standard English lower-case alphabet of 26 letters 'a', 'b',.....,'z' to form at least 300,000 words of equal length where no letter is repeated within a word.

What is the minimum length word that will suffice?

(c) In how many ways can a group of thirteen people be divided into two groups containing five people and a third group containing three people?

My english is weak, can someone explain to me how to do (a) and (b) please! thank you

for (c), it is very confused me, why there has a third group??