1. **Problem:** A bag contains 10 black balls and 15 white balls. If a ball is picked at random without replacement, what is the probability of picking a white ball? 2. **Formula:** Probability of an event = $ \frac{\text{Number of favorable outcomes}}{\text{Total number of outcomes}} $ 3. **Step 1:** Calculate total number of balls: $$10 + 15 = 25$$ 4. **Step 2:** Number of favorable outcomes (white balls) = 15 5. **Step 3:** Calculate probability: $$P(\text{white ball}) = \frac{15}{25}$$ 6. **Step 4:** Simplify the fraction: $$P(\text{white ball}) = \frac{\cancel{15}}{\cancel{25}} = \frac{3}{5}$$ 7. **Answer:** The probability of picking a white ball is $ \frac{3}{5} $. Hence, the correct choice is [B] 3/5.