Question 1208344
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Note there is no need to change the question that is asked.  If Alan took some marbles, then Ben took some, and then Caleb took the ones that were left, then there were no marbles left.  So asking how many marbles there were at first is the right question.<br>
The algebra involved if you work this kind of problem in the "forwards" direction can get a bit ugly.  Often a problem like this can be solved more easily by working backwards.<br>
Caleb took the last 12 marbles.  The last thing that happened before that was Ben took 3 marbles, so before Ben took those 3 marbles the number of marbles was 12+3 = 15.<br>
The last thing that happened before Ben took those 3 marbles was he took 25% (1/4) of the marbles.  So the 15 marbles before Ben took his last 3 are 75% of the number there were before Ben took any.  So 100% of the number before Ben took any was 20.<br>
Before Ben took any, the last thing that happened was Alan took 1 additional marble; so before he took his last one the number of marbles was 20+1 = 21.<br>
And before Alan took his last one, he took 40% of the original number of marbles.  So the 21 marbles after he took 40% of the original number are 60% of the original number, which means 100% of the original number of marbles was 35.<br>
With all the words of explanation, that sounds like a very long solution.  But if your mental math is good, it doesn't take much time.<br>