The charge Q during charging of the capecitor is given by
Q = Qo ( 1 e^t/T). In your question Q = 75% 0f Qo, or Q= (3/4)Qo,hence
3Qo/4 = Qo (1e^t/T)
3/4 = 1  e^t/T
e^t/T = 1 3/4 = 1/4
e^t/T = 4 ( taking reciprocal both the sides )
Now take natural log (ln)
t/T = ln4
t = T*2ln2.
Now T, the time constant should have been given in the question.
Or as T = RC, R should have been given.
