A fruit vendor is stocking apples and bananas. The vendor wants to spend no more than $50$ total. Apples cost $1.50$ per pound, and bananas cost $0.75$ per pound. The vendor wants to buy at least 3 times as many pounds of apples as bananas. Let $a$ represent the number of pounds of apples and $b$ represent the number of pounds of bananas, where $a$ and $b$ are nonnegative values. Which of the following systems of inequalities best represents this situation?