Now        70
30 min     35
60 min     17.5
90 min     8.75

The condition says

V(t) = ,

where V(t) is the current volume,  is the initial volume and t is time in hours.


In 1.5 hours,  v(1.5) = 70 : 2^(2*(3/2)) =  = 8.75 cibic meters.




Or, 3/2 hours is 3 times (1/2) of an hour.


So, divide 70 by 2 three times.


You will get the same answer.