Explanation:
as a first step we can simplify the ratio (because it is nothing else than a fraction) :
56:22 = 28:11
it means that for every 28 movie posters there should be 11 band posters.
now, when she sells 28 movie posters (that is actually half of her stock), she has 28 movie posters left.
that did exactly the same operation we already did at the beginning with the simplification. so, we know already the answer.
but what, if it would not be such a nice, easy number ?
so let's do it formally fully correct :
as a ratio is the same as a fraction, the same operations apply. if we change one number by a factor, we need to multiply the other number by the same factor to keep the overall value unchanged.
56 × f = 28
f = 28/56 = 1/2
=>
22×f = 22×1/2 = 11
so, she wants to keep 11 band posters to keep the same ratio. and that means she has to sell 11 band posters.