If a, b, c, and d are positive real numbers such that a + b + c + d = 10, which of the following could be the value of a + c?
Correct: E
We know that 0 < a < 10, 0 < b < 10, 0 < c < 10, and 0 < d < 10, and a + b + c + d = 10. It follows that a + c < 10. However, since b and d can both be made arbitrarily close to zero, a + c can be made to approach 10. So the correct answer is any of the options less than 10.