Rennae needs to purchase at least 20 boards for group art project. The poster boards come in the small size, which costs $30 per board, and the larger size., which costs $50 per board. Her allowance for purchasing poster boards is no more than $860 in total. She must purchase at least 5 small poster boards and 4 larger poster boards.If a represents the number small poster boards and b represents the number of large poster boards, which of the following system of inequalities represents the restrictions described?