Final answer:
To have enough roast beef for dinner and lunch, you should buy 4 pounds.
Step-by-step explanation:
To determine how much roast beef you should buy, you need to calculate the total amount of roast beef needed for dinner and for making sandwiches. Since you want enough roast beef for 6 people for dinner and 4 sandwiches for lunch, you need to calculate the total amount of roast beef for both meals.
Let's say each person needs 1/2 pound of roast beef for dinner. So, for 6 people, you would need 6 * (1/2) = 3 pounds of roast beef for dinner.
For the sandwiches, let's assume each sandwich requires 1/4 pound of roast beef. So, for 4 sandwiches, you would need 4 * (1/4) = 1 pound of roast beef for lunch.
Therefore, the total amount of roast beef you should buy is 3 pounds + 1 pound = 4 pounds.