Starting with a sample of pure Cu66,7/8 part of sample decay’s into Zn in 21 minutes. The  corresponding  mean-life is

If 78 part decayed, then remaining sample is 18. Decay Law, NN0=12tt12 ⇒18=12tt12 ⇒123=12tt12 ⇒3=tt12 ⇒t12=213min=7min  Now, Mean Life= Half lifeln2=70.693=10.10  min