What is the probability of a person having exactly 1 sibling ?

What is the axis of symmetry of the function f(x)=-(x+ 9)(x-21)

What’s the range cuz i can’t find it?

Astudent must have an average on five test that is greater than it equal to 80% but less than 90% to receive a final grade of b. devon's greades on the first four test were 78% 62% 91% and 80% what range if grades on the fifth test would give him a b in the course? ( assuming the highest grade is 100%)