half life period of a substance is 50 min at certain initial concentration, when concentration is reduced to half of its initial concentration , the half life is reduced to 25 min. calculate the order

It is given that the half life of the compound is reduced to half (50 mins. to 25 mins.) when initial concentration is reduced to half. This means that half life is directly proportional to initial concentration.
So, the order of the reaction is zero.