Question 762862
72x+50y=9xy ..............1
90x+70y=12xy..............2
now dividing both eqns by xy on both sides we will get
72/y+50/x=9..........3
90/y+70/x=12.........4
multiplying eqn 3 by 10 & eqn 4 by 8 we get
720/y+500/x=90.......5 
720/y+560/x=96
-    -      -
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-60/x=-6
or x=10
putting this value in eqn 3
72/y+50/10=9
or 72/y=9-5=4
or y=72/4=18