A bag holds 20 balls, some of them red. If 10 more red balls are put in, the probability of drawing…

Step 1. The equation. At first $P = \dfrac{x}{20}$. After adding, $P = \dfrac{x + 10}{30}$. So $\dfrac{2x}{20} = \dfrac{x + 10}{30}$.

Step 2. Solve. $60x = 20x + 200$, so $40x = 200$ and $x = 5$.

Step 3. Check. At first $\dfrac{5}{20} = \dfrac{1}{4}$. After adding, $\dfrac{15}{30} = \dfrac{1}{2}$, which is double, and equals $0.5$.