My approach

let there be 1% alcohol

Let the volume be 100x

99x water 1x alcholol

let the new volume be 100y

98y water and 2y alchol

now we know that water evaporated so this gives the volume of alchohol remains the same.

so 1x= 2y

x = 2y

water evaporated is 99x-98y = 49x

But the answer isSPOILER: 50.5%