The reaction L→M is started with 10.0g 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 12 therefore t1/2 is 30 minutes. In 90 minutes the amount is reduced 18i.e.1(2)n. Here n=3 . True for 1st order reaction.