The friction of the air causes a vertical retardation equal to 10% of the acceleration due to gravity (take g = 10 ms-2). The maximum height will be decreased by:

Retardation due to friction of air = g10. Hence in upward motion:total retardation = g+g10 = 11g10Hm = u2sin2θ2gHm' = u2sin2θ2×11g10 = 1011×u2sin2θ2g= 1011Hm% decreases in Hm= Hm-Hm'Hm×100                                = (1-1011)×100 = 9%