Maths Olympiad Prep

Library / /265 of 520

Combinatorics Difficulty 3.1 AMC 10/12 Find the answer

Six bags of marbles contain 18,19,21,23,2518, 19, 21, 23, 25 and 3434 marbles, respectively. One bag contains chipped marbles only. The other 55 bags contain no chipped marbles. Jane takes three of the bags and George takes two of the others. Only the bag of chipped marbles remains. If Jane gets twice as many marbles as George, how many chipped marbles are there?

Pick one

Solution

Let the number of marbles George has be xx, and so the number of marbles Jane has is 2x2x. Therefore, the total number of non-chipped marbles is 3x0 (mod 3)3x\equiv0 \ (\text{mod }3). However, the total number of marbles is 18+19+21+23+25+34=1402 (mod 3)18+19+21+23+25+34=140\equiv2 \ (\text{mod }3). Therefore, we need a number of chipped marbles 2 (mod 3)\equiv2 \ (\text{mod }3) to get a number of non-chipped marbles 0 (mod 3)\equiv0 \ (\text{mod }3).
180 (mod 3)18\equiv0 \ (\text{mod }3)
191 (mod 3)19\equiv1 \ (\text{mod }3)
210 (mod 3)21\equiv0 \ (\text{mod }3)
232 (mod 3)23\equiv2 \ (\text{mod }3)
251 (mod 3)25\equiv1 \ (\text{mod }3)
341 (mod 3)34\equiv1 \ (\text{mod }3)
Since 2323 is the only one 2 (mod 3)\equiv2 \ (\text{mod }3), it is the only possible number of chipped marbles, D\boxed{\text{D}}.
To check, we can see that if Jane takes the 19,25,19, 25, and 3434 marble bags and George takes the 1818 and 2121 marble bags, this satisfies the conditions.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.