An alloy of zinc and tin contains 37% zinc by weight. Find the weight of zinc which must be added to 400 kg of this alloy if the final percentage of zinc is to be 70.

Explanation

Total weight of alloy `= 400 kg`

Let we add `x` kg zinc to make its weight `70%` in the alloy.

Then, total alloy `= 400+x`

Total zinc after adding `x` kg` = 37**400/100+x`

`:. (400+x)**70/100 = (37**4)+x`

`=>2800+7x = 1480+10x`

`=>3x = 1320=> x = 440kg`

So, zinc of weight of `440` kg should be added to get the desired mixture.