Question 1196652
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Presumably the intent of the problem was that you solve it using formal algebra; the other tutor showed one typical formal solution.<br>
You can get good problem-solving practice by solving the problem informally, using logical reasoning and simple mental arithmetic.<br>
(1) Set aside the 3 "extra" marbles that Don has.  That leaves 96 other marbles, with Don having 3 times as many as Mark.
(2) That means Mark has 1/4 of those remaining 96 marbles (24), and Don has 3/4 of them (3*24 = 72).
(3) Now give Don's "extra" 3 marbles back to him, giving him 72+3 = 75.<br>
ANSWER: Mark has 24 marbles; Don has 75.<br>