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Bunuel
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Bunuel
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understood it up till the LCM. why do we need it and what is it for?
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rohanbnj
understood it up till the LCM. why do we need it and what is it for?


 
We need it to find the common terms of 9b+4 and 4a+3. We saw that the least common term is 31. If I further find the other terms for both, we will see that the next common is 67. Therefore, the common difference between 67 and 31 is 36. But we will not spend so much time to find all the common terms between the two expressions. Rather I will take the LCM of the coefficients and add it to the first term. The series thus becomes 31,67,103,.. and so on. Hope this helps.­
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Bunuel
­Students in a class are arranged to form groups of 4 members each. After forming the groups, 3 students are left. If the students had been arranged in groups of 9 members each, however, 4 students would be left. What is the total number of students in the class?

(1) The number of students is a two-digit number less than 65.
(2) The number of students is a two-digit number greater than 30.


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If I consider that the total number of students is 'n' then if I take groups of 4, I will be left with 3 more students. So I can say that I have a total of 4n+3 students.
Similarly, for groups of 9, I will have a total of 9n+4 students.
Now, put n=1,2.. and find the values for each expression.
4n+3 will yield 7,11,15,19,23,27,31,35,39,43,47,51,55,59,63,67....
9n+4 will yield 13,22,31,40,49,58,67,...
What is the least common term? 31. Then we have the second common term i.e. 67. Therefore, the series becomes 31,67,....
[I showed each term, you can simply take the LCM of the coefficients of both expressions i.e. 4 and 9. You will get 36 and keep adding it with each term to find all possible common terms. It will be 31,67,103,... and so on.]

We now have 31,67,....and so on.
1. if n<65, only one value is possible, i.e. 31. Sufficient. Keep A and D.
2. n>30 so it can be 31,67,... Insufficient. Remove D.
Option (A) is correct.
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