there is a 72,000 income from three investments, therefore the 1/2 of the investments is 36,000. to get the interest of the stock.
we divide 36,000 by 100% then multiply it by 9%= 3240
1/4 of the investments for mortgages is 18,000 to get the interest of the stock.
we divide 18,000 by 100% then multiply it by 8%=1440
the investments left is the 1/4 of the 72,000 to get bonds.
we divide 18,000 by 100% then multiply it by 6%=1080