But randomly choosing two students from the first class made up of 21 students one time out of two the chosen student wears glasses what is the number of students in the first class who bring eyeglasses

answer: 15

can someone explain?

hi

we need to find the probability that both students chosen will wear glasses

P = x/21 * (x-1)/20 = 1/2

x² - x - 210 = 0

b²-4ac = 841

square root 841 = 29

solutions = (1-29)/2 = - 14 not a possible answer

or solution = (1+29)/2 = 15

so the solution must be 15 students