For a first order reaction, t1/2 = 0.693/k ⇒ k = 0.693/25 = 0.028 min-1
Now using first order equation, k = (2.303/t)log10[R0]/[R] ⇒ t = (2.303/0.028)log10[R0]/(0.2[R0]) (∵[R] = 100% - 50% = 50% = 0.5[R0])
t = 82.25 log105 = 82.25 × 0.699 = 57.49 min