At any instant the ratio of the amount of radioactive subatance is 2:1.if there half lives be respectively 12 and 16 hours,then after two days what will be the ratio of the substances?

You could work it up formulaically, but note that 48 hours is conveniently an integer multiple of both 12 and 16 hours.

After 48 hours, the first substance, for example, will have "halved" itself 4 times, resulting in a quantity that's \left( \frac{1}{2} \right)^4 of its initial quantity.

Also, let 2x and x denote the substances' initial quantities.

Take it from here?