Mr. Jones has $18,000 to invest in three types of stocks, low-risk, medium-risk, and high-risk. He invests according to three principles. The amount invested in low-risk stocks will be at most $4,000 more than the amount invested in medium-risk stocks. At least $8,000 will be invested in low- and medium-risk stocks. No more than $14,000 will be invested in medium- and high-risk stocks. The expected yields are 6% for low-risk stocks, 7% for medium-risk stocks, and 8% for high-risk stocks. How much money should Mr. Jones invest in each type of stock to maximize his total expected yield? Mr. Jones should invest $ in low-risk stocks, $ in medium-risk stocks, and $ in high-risk stocks