7 elementary classes will be chosen to participate in a school lunch survey. There are 21 elementary classes in the school. In how many ways can the classes be chosen?
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( About ) 1.01371e + 16 ways

Step-by-step explanation:

You have 21 choices for the first student chosen, 20 choices for the next, then 19, then 18. However, this over-counts everything by a factor of 7! (the number of ways in which four objects can be arranged with regard to order).

Thus, take a look at the procedure below;

This is an approximate answer mind you, so I hope that still helps!