A drum has a combination of 2 liquids X and Y in the proportion 7:5 when 9 L of combination are taken out and the Drum is filled with Y, the proportion of X and Y becomes 7:9. Then how many litres of liquids X was obtained by the Drum initially?
Choose one:
A) 21 litres B) 22 litres C) 23 litres D) 24 litres E) 25 litres
Explanation :   =&gt; If the Drum has 7a and 5a litres of combinations X and Y correspondingly. 21 litresmount of Combination taken out is 9 litres   Some of X in Combination left = [ 7a - (7/12)*9] litres   = [ 7a - (21/4) ] litres   Some of Y in combination left = [ 5a - (5/12)*9 ] litres   = [ 5a - (15/4) ] litres   =&gt; Since the combination is in the ratio 7:9,   { [7a - (21/4)] / [ 5a - (15/4) ]+9 } = (7/9) =&gt; [ (28a - 21) / (20a + 21) ] = (7/9)   =&gt;252a – 189 = 140a + 147   =&gt; 112a = 336   =&gt; a = 3   Some of liquid X, has in the Drum initially is (7*3)= 
<gadget id="calculator">(7*3)</gadget>
<output>21</output>
21 litres   21 litresnswer : 21 litres
<result>A</result>
A drum has a combination of 2 liquids X and Y in the proportion 7:5 when 9 L of combination are taken out and the Drum is filled with Y, the proportion of X and Y becomes 7:9. Then how many litres of liquids X was obtained by the Drum initially?