Mr. Jones has $9000 to invest in three types of stocks; low-risk, medium-risk and high-risk. He invests according to the three principles. The amount invested in low-risk will be at most $1000 more than the amount invested in medium-risk stocks. At least $5000 will be invested in low and medium-risk stock. no more than $7000 will be invested in medium-risk and high-risk stocks. The expected yields are 6% for low-risk, 7% for medium-risk and 8% for high-risk stocks. How much money should Mr. Jones invest in each type of stock to maximize his total expected yield.