Lets say he had x shares

And the price of each share = y

Total value for all stocks = 720$ = xy

Thus our first equation is

xy= 720$

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After a month

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No. shares sold =x-20

rice of each share = y+ 8

Amount he received =(x-20)(y+8) = 800$

Thus our second equation is

(x-20)(y+8) = 800$

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Solve the two equations

x = 60 , y= 12