For two real values of $n$, the equation $4x^2+nx+25=0$ has exactly one solution in $x$. What is the positive value of $n$?
I notice that this is a quadratic equation, and I know that it has exactly one solution in $x$ when the discriminant is zero. The discriminant is the part under the square root in the quadratic formula, so I can write it as $n^2-4(4)(25)$. Setting this equal to zero, I get $n^2=400$, which has two real solutions: $n=\pm 20$. The problem asks for the positive value of $n$, so I choose $n=20$. 