For first one, u can think of it this way imo..
We need the last digit of the number as the fractional part should be same . Now as the period of last digit being same for 3^nat.no is 4
It will be periodic with T=4 ..
Eg 0.1,0.3,0.9,0.7, 0.1
For second one is it 2? Not fully sure tho. @Siddhant_Mudholkar