The friction of the air causes a vertical retardation equal to 10% of the acceleration due to gravity. Take g = 10 m/s2. The maximum height and time to reach the maximum height will be decreased by

h=u2sin2⁡θ2g′=u2sin2⁡θ2×1110g=1011×u2sin2⁡θ2g=u2sin2⁡θ2g×1011×100%=91% of u2sin2⁡e2gSo, maximum height decreases by 9%Further, t=usin⁡θg′=usin⁡θ(11/11)g=91% of usin⁡θg∴    time to reach the maximum height decreases by 9%.