John has made $250 in 10 days, so this means he makes $250/10=<<250/10=25>>25 dollars a day.
Since John wants to make double his current amount, this means he wants to make 250*2=<<250*2=500>>500 dollars
This means John needs to work 500/25= <<500/25=20>>20 days in total to make 500.
As John has already worked 10 days, this means he needs to work 20-10=<<20-10=10>>10 more days to make his goal.
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