Question 314848
Originally you had {{{4N}}} balls, {{{N}}} of them were red. 
{{{5}}} red balls and {{{35}}} non-red balls are now added. 
The total number of red balls is now {{{N+5}}}
The total number of balls is {{{4N+40}}}.
(1/5) of the new total are red. 
{{{(1/5)(4N+40)=N+5}}}
{{{4N+40=5N+25}}}
{{{N=40-25}}}
{{{highlight(N=15)}}}
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Verify.
You originally had 15 red balls, and 60 total balls (15/60=1/4).
Then you added the additional balls.  
You then had 20 red balls and 100 total balls (20/100=1/5).