If retardation produced by air resistance of projectile is 1/10 of acceleration due to gravity, the time to reach maximum height

We know that,T=usin⁡θ/gIf a is retardation caused due to air resistance, then total retardation, a' = (g + a). NowT′=usin⁡θ(g+a)Substituting a = (g / 10), we getT′=usin⁡θ[g+(g/10)]=10usin⁡θ11g=1011TThe percentage decrease in time is given byT−T′T×100=T−(10T/11)T×100=9%