A)The total cost would be: x=15a+12b+18c where x is the total cost and a,b,and c are the amount of stocks you bought from their respective companies.
B) Since you bought 10 A stocks, you must have bought 20 C stocks. That adds up to 30 bought, leaving 15 for B.
x=(15)(10)+(12)(15)+(18)(20)
x=150+180+360=$690 spent
C) A's stock is now 15(1.25) which is 18.75. C's stock is now 18(.90) which is 16.2
new equation is 18.75a+12b+16.2c=x
D) Substitute the number of stocks we bought from each in again:
187.5+180+324=$691.5
The cost went up slightly, so divide 690/691.5 then multiply that number by one hundred. You get that you spent in B 99.8% the amount you spent in C. So 100-99.8= an increase of .2%