Let $X = Y_1 + Y_2$. Find the conditional probability $P[Y_2\ge4|X\le7]$. Round your answer to 3 digits.
By drawing a chart for $X=Y_1+Y_2$, we see that $P[Y_2\ge4, X\le7] = 6/36$ and $P[X\le7] = 21/36$ So we get $P[Y_2\ge4|X\le7] = 2/7$ which I rounded to $0.29$. I don't understand why but the Coursera course I'm taking won't accept this.