During one day on the stock market, an investor lost $2500 on one stock, but gained $1700 on another. at the end of trading that day, the investor's holdings were worth $52500. what were they worth when the market closed that day?

Let x and y represent the two holdings.

Before trading day:
The holdings were worth x + y.

At the end of the trading day:
Holding x  changes to x - 2500 because of loss.
Holding y changes to y + 1700 because of gain.

The holdings are worth $52500 at the end of trading, therefore
(x - 2500) + (y + 1700) = 52500
x + y - 800 = 52500
x + y = 53300

Before the trading day, the holdings were worth $53,300.
At the end of the trading day, the holdings were worth $52,500.

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