1. The problem states that Shasta's weight $p$ in pounds varies with age $a$ in months according to the equation $$p = a + 3$$. 2. We are asked to find Shasta's weight when $a = 10$ months. 3. Substitute $a = 10$ into the equation: $$p = 10 + 3$$ 4. Simplify the right side: $$p = 13$$ 5. Therefore, Shasta's weight at 10 months old is 13 pounds.