If T is the time to reach maximum height in absence of air resistance then

**T = u sinθ / g**

If retardation is also considered then, then it will add to acceleration due to gravity

**i.e. g → g + a**

T' = u sinθ / g + a

**= **u sinθ / g + (g/10)

= 10u sinθ/ 11g

= 10/11 T

percentage decrement = (actual decrement / actual time taken)x 100

= [(T - 10/11T)/ T] x 100

= 1/11 x 100 = **9.09 %**