since they said "at least", technically we want inequalities here, but i'll use equations.

Let be the amount, in pounds, of Kona beans we add to the coffee to make it a Kona Blend. then at the end, we will have pounds of Kona Blend. now we want the amount of Kona Beans to be 30% of the mixture of the Kona Blend, thus we want:

now solve for