Question 187624
"Mike invested $6000 for one year" ----> x+y=6000 ----> y=6000-x



"He invested part of it at 9% and the rest at 11%. At the end of the year he earned $624 in interest" ----> 0.09x+0.11y=624 ----> 9x+11y=62400



9x+11y=62400


9x+11(6000-x)=62400


9x+66000-11x=62400


-2x+66000=62400


-2x=62400-66000


-2x=-3600


x=1800



He invested $1,800 at 9%


y=6000-x


y=6000-1800


y=4200



He invested $4,200 at 11%