Initial amount of sugar in his diet is 15 spoons in a day. Currently, the amount of sugar in his diet is 9 1/2 spoons in a day. Converting to decimal, it becomes 9.5 spoon in a day. The amount by which he reduced his sugar intake is 15 - 9.5 = 5.5 spoons.
Percentage decrease would be expressed as
Amount by which the intake decreased/ initial intake × 100
