If y varies directly as x, what is the value of y in these ordered pairs? (5, 8) and (10, y)

Direct variation describes - roughly - a relationship between two values where scaling one value will scale the other by the same amount. The x value doubles as it goes from 5 to 10, and since y varies directly as x, we can double the old y value to get the new one as well. Since the old y value is 8, our new y value should be 8 x 2 = 16.