.
In all, there are 4 + 6 + 7 + 3 = 20 students.
By selecting randomly 3 students of 20, you can do it in = = 1140 ways.
In how many ways can you select the group of 3 juniors ?
You select them from the sub-group of 7 junior students, therefore, you can do it in = = 35 ways.
Therefore, the probability under the question is this number = 0.031 = 3.1%.
In order to solve freely such problems, you need to know about Combinations.
On Combinations, see the lessons
- Introduction to Combinations
- PROOF of the formula on the number of Combinations
- Problems on Combinations
- OVERVIEW of lessons on Permutations and Combinations
in this site.
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic "Combinatorics: Combinations and permutations".
Save the link to this textbook together with its description
Free of charge online textbook in ALGEBRA-II
https://www.algebra.com/algebra/homework/complex/ALGEBRA-II-YOUR-ONLINE-TEXTBOOK.lesson
into your archive and use when it is needed.