If x+4 is a factor of x^2+bx+20, what is the value of b?

If a factor is x+4, that means that the coefficient of x^2 (the number in front of it, which would be 1) and that 20 should be divisible by 20. Now, if you divide 20 by 4, you would get 5, and 1/1 = 1. So, the 2 factors you have is

(x+4)(x+5)

When you FOIL, you would get
x^2+9x+20, which means

b=9