Question

The cost price of 10 articles is equal to the selling price of 9 articles. Find the gain %

Answer = 100/9% gain

Answer 1

Solution is in the attached image..

Answer 2

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Here is your answer
Let the CP of 10 Articles is x
CP of 1 Article is x/10

So SP of 9 Articles = CP of 10 Articles = x
SP of 1 Article is x/9

Profit = SP - CP
$= \frac{x}{9} - \frac{x}{10} \\ = \frac{10x - 9x}{90} \\ = \frac{x}{90}$
Now we know
Profit % =( Profit× 100)/CP

$= \frac{ \frac{x}{90} }{ \frac{x}{10} } \times 100 \\ = \frac{x}{90} \times \frac{10}{x} \times 100 \\ = \frac{100}{9} \%$

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