x -> her money;
( 1 / 4 )*x = x / 4 for a pair of shoes;
( 1 / 5 )*x = x / 5 for a shoulder bag;
x - x / 4 -x / 5 = 20x / 20x - 5x / 20x - 4x / 20x = 11x / 20 the remainder;
11x / 20 - ( 3 / 8 )*x = 11x / 20 - 3x / 8 = 22x / 40 - 15x / 40 = 7x / 40 for her dress;
She pay 7 / 40 of her money for the dress.