Word problem help: An investor owns a portfolio of three stocks, X, Y, and Z...?

On March 1, the closing prices were X, $44; Y $22; and Z $64. The value of the investor's portfolio amounted to $20, 480. Three weeks later, the closing prices were X, $42; Y, $28; and Z, $62. the investor's portfolio was valued at $20,720. Two weeks later, the closing prices were X, $42; Y,$30; and Z, $60 - with a total portfolio value of $20, 580. How many shares of each stock did the investor hold in the portfolio?