Question 1185640
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I don't think the "similar" problem solved by the other tutor is equivalent to your problem....<br>
The wording of the problem implies that all four different types of amino acids are used in the chain of 5 amino acids.  So we are arranging the symbols AABCD.<br>
(1) We first need to choose 1 of the 4 amino acids to be the one that occurs twice: {{{C(4,1)=4}}}.<br>
(2) Then we can arrange the 5 amino acids, of which 2 are the same, in a chain: {{{5!/2! = 60}}}<br>
ANSWER: 4*60 = 240 different strings of the four amino acids are possible if the string contains all of the four types of amino acids and two are the same.<br>