The reaction $L\to M$is started with 10.0 g of L. After 30 and 90 minutes 5.0 g and 1.25 g of L

respectively are left. The order of the reaction is  

After every 30 minutes the amount is reduced to $\frac{1}{2}$ therefore $t_{1/2}$is 30 minutes. In 90 minutes the amount is reduced to $\frac{1}{8}i.e.\frac{1}{2^n}$. Here n = 3. True for 1^st order reaction.