Let the alloy contain $x \Kg$ silver and $y \kg$ copper Now when mixed with $3\Kg$ Pure silver we get : ${x+3} / {x + y+ 3} = 9/10$ we get $10x + 30 = 9x + 9y + 27$ $9y - x = 3$
...(1)
Now as per condition 2 silver in $2^{nd}$ alloy $= 2(0.9) =1.8 $ So we get - ${x+1.8} / {x + y+ 2} = 21/25$ we get $21y -4x =3$
...(2)
solving (1) and (2) we get $y= 0.6$ and $x =2.4$ so $x + y = 3$