Which of the following represents a null set

Astore sells two types of radios. one type sells $87 and the other for $119. if 25 were sold and the sales were $2495, how many of the $87 radios were sold a) 5 b) 20 c) 15 d)10

F(n + 1) = f(n) – 8. if f(1) = 100, what is f(6)?

What is the y-intercept of the line given by the equation