Expert: Scott A Wilson - 11/3/2009

Question
A principal of $2100 was invested in the stock market. Some shares of World Oil were bought at $7.50per share. Some shares of Zinco Mines were bought at $3.25per share.  The total number of shares was 450. How many shares of each stock were bought?

Answer
Let the shares of World Oil be W.
Let the shares of Zinco Mines be Z.

What we are given is that $7.5•W + $3.25•Z = $2,100.
We also know that W + Z = 450, so W = 450 - Z.
Putting this into the 1st equation gives $7.5•(450 - Z) + $3.25•Z = $2,100.

Multiplying this out gives $3375 - $7.5Z + $3.25Z = $2,100.
Combining the Z's gives $3375 - $4.25Z = $2,100.

Addine $4.25Z - $2100 to both sides gives $1,275 = $4.25Z

This tells us that Z was 1,275/4.25 = 300.

Since the sum of shares is 450, that means W is 150.