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