Tom buys 20 shares of a stock that costs $3 each.  He sells 10 of those shares for $4 each.  The rest of the shares end up doubling in value.  How much profit does he make?
He spends 3*20=$<<3*20=60>>60 on the shares.
He sells 10 for 10*4=$<<10*4=40>>40.
He sells the other 20-10=<<20-10=10>>10 for a higher valuation.
Their price was 3*2=$<<3*2=6>>6.
So he got 10*6=$<<10*6=60>>60 from those.
So in total, he got 60+40=$<<60+40=100>>100.
So his profit was 100-60=$<<100-60=40>>40.
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