A customer enters a supermarket. The probability that the customer buys bread is .60, the probability that the customer buys milk is .50, and the probability that the customer buys both bread and milk is .30. What is the probability that the customer would buy either bread or milk or both?
1. **Define the events and given probabilities:**
   - Let \( B \) be the event that the customer buys bread.
   - Let \( M \) be the event that the customer buys milk.
   - We are given:
     \[
     P(B) = 0.60, \quad P(M) = 0.50, \quad P(B \cap M) = 0.30
     \]

2. **Use the principle of inclusion-exclusion to find the probability that the customer buys either bread or milk or both:**
   - The principle of inclusion-exclusion for two events states:
     \[
     P(B \cup M) = P(B) + P(M) - P(B \cap M)
     \]

3. **Substitute the given probabilities into the formula:**
   \[
   P(B \cup M) = 0.60 + 0.50 - 0.30
   \]

4. **Perform the arithmetic operations:**
   \[
   P(B \cup M) = 0.60 + 0.50 - 0.30 = 0.80
   \]

5. **Interpret the result:**
   - The probability that the customer buys either bread or milk or both is \( 0.80 \).

The final answer is \( \boxed{0.80} \)