Cuales valores de z hacen que la expresión no esté definida?

Ahiker stands at an unknown elevation before he ascends 2 miles up a mountain to an elevation of 9 miles. find the elevation of the hiker before his ascent.

Aclassroom is made up of 11 boys and 14 girls. the teacher has four main classroom responsibilities that she wants to hand out to four different students (one for each of the four students). if the teacher chooses 4 of the students at random, then what is the probability that the four students chosen to complete the responsibilities will be all boys?

What is the solution for 6 - 7/8? explain the steps.