A particle undergoes simple harmonic motion having time-period T ( Starting from the mean position). The time taken for 3/8th oscillation is-

38th oscillation means 28+18=14+18 starting from mean position 14 oscillation means it will go from mean position to extreme position  and from extreme position it has travelled half the amplitude i.e 18th oscillation. from extreme position to half the amplitude a2=acosωt t=T6  so total time T4+T6=5T12