A body is oscillating simple harmonically with a period T = 2 s. How much time  would it take for its kinetic energy to decrease from Kmax to 75 % of Kmax ?

K=12mv2=m2A2ω2cos2(ωt) ⇒Kmax=mA2ω22 ⇒by question, K=75%Kmax ⇒m2A2ω2cos2(ωt)=75100mA2ω22 ⇒cosωt=32 ⇒cosωt=cosπ6 ⇒ωt=π6 ⇒2πTt=π6 given T=2sec substiitute ⇒2π2t=π6 ⇒t=16sec