Phil bought 700 shares of a company's stock for $9.29/share. he pays a broker a commission of $18 to buy and sell stock. after one year, he sold all his shares, which were worth $9.90/share at that time.
a) 6.0%
b) 6.2%
c) 6.5%
d) 6.8%

It seems that you are looking for the yield of the shares of stocks after they were sold.
To solve, 
yield =( current total amount- previous total amount) / previous amountwherecurrent amount = $9.9/share*700share+ $18previous=$9.29/share*700 share
yield=[ (9.9*700)+18] -(9.29*700)/(9.29*700) x 100
yield= 6.8%
Shares of stocks yield = 6.8%

When rates are expressed as a quantity of 1, such as 2 feet per second or 5 miles per hour, they are called unit rates. if you have a multiple-unit rate such as 120 students for every 3 buses, and want to find the single-unit rate, write a ratio equal to the multiple-unit rate with 1 as the second term.