The probability that the amount of marbles in the red box (of total five boxes) will be no more than 523. ( Total amount of marbles - 2500).

Question

The total amount of marbles - $2500$. There is only one red box out of 5 boxes. I have to calculate the probability that the amount of marbles in the red box will be no more than 523. My solution ( I am using Gaussian integral theorem). $F(z)=\frac{1}{2\pi} \cdot \int_\infty^{z} e^{-\frac{t^2}{2}}.$ The values for the $z$ can be found in Gaussian table, maybe some of you are familiar with it? $$F\left(\frac{523-500}{20}\right) - F\left(\frac{0 - 500}{20}\right) = F\left(1,15 \right) - F\left(-25 \right)=F(1,15) = 0,3749.$$ Is my solution correct?