Permutations and Combinations: Mathematics | JEE Main
A boy needs to select five courses from 12 available courses, out of which 5 courses are language courses. If he can choose at most two language courses, then the number of ways he can choose five courses is
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Hint 1 of 4
How can the condition of selecting 'at most two language courses' out of five total courses be partitioned into mutually exclusive cases?
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Correct answer
The number of ways to choose five courses such that at most two are language courses is 546.
Option analysis
Why each option works or fails
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Solution
StepWorking
01given
Total courses = 12, of which 5 are language courses and 12−5=7 are non-language courses. We need to select 5 courses in total with at most 2 language courses.
02goal
Find the total number of ways to choose 5 courses satisfying the condition of selecting at most 2 language courses.
03approach
Split the requirement 'at most 2 language courses' into three mutually exclusive cases: 0 language courses, 1 language course, and 2 language courses. For each case, choose the remaining required courses from the 7 non-language courses and sum the combinations.
04execute
Case 1: 0 language and 5 others ⟹(05)(57)=1×21=21.
Case 2: 1 language and 4 others ⟹(15)(47)=5×35=175.
Case 3: 2 languages and 3 others ⟹(25)(37)=10×35=350.
Total ways =21+175+350=546.
✓verify
Using complementary counting: Total ways without restrictions =(512)=792. Unfavorable cases: 3 language courses ⟹(35)(27)=10×21=210; 4 language courses ⟹(45)(17)=5×7=35; 5 language courses ⟹(55)(07)=1×1=1. Total unfavorable =210+35+1=246. Desired ways =792−246=546, which matches.
Hints that build this answer step by step
How can the condition of selecting 'at most two language courses' out of five total courses be partitioned into mutually exclusive cases?
0 language courses, 1 language course, or 2 language courses
Given 5 language courses and 7 non-language courses, which sum of combinations represents the three valid cases?
C(5,0)*C(7,5) + C(5,1)*C(7,4) + C(5,2)*C(7,3)
What are the numerical values of the three terms C(5,0)*C(7,5), C(5,1)*C(7,4), and C(5,2)*C(7,3)?
21, 175, and 350
What is the total number of ways to choose the five courses?
Why do we multiply within each case and add across cases?
Within a case, choosing languages and non-languages are independent sequential choices (multiplication principle). The three cases are mutually exclusive alternatives (addition principle).