Question 369440
{{{P(Y)=0.2}}}
Let Y be the number of Y cards added to the box.
There were 9 cards to start with in the box.
{{{P(Y)=Y/(Y+9)=2/10}}}
{{{10Y=2(Y+9)}}}
{{{10Y=2Y+18}}}
{{{8Y=18}}}
Since {{{8}}} is not a factor of {{{18}}}, there is no integer answer. 
{{{Y}}} must be an integer.
Please check the problem setup.