Can someone please help me with this question..

I attempted it and got the correct ans for the first 3 parts but for the last part my ans was wrong, and I couldn't figure out why

One end of a piece of elastic is attatched to a point at the top of a door frame and the other end hangs freely. A small ball is attatched to the free end of the elastic.When the ball is hanging freely it is pulled down a distance and then released, so that the ball oscillated up and down on the elastic.The depth d centimetres of the ball from the top of the door frame after t seconds is given by:

d= 100+10cos 500t degrees

Find:

a) the greatest and least depths of the ball

b)the time at which the ball first reaches its highest position

c)the time taken for a complete oscillation

d)the proportion of the time during a complete oscillation for which the depth of the ball is less than 99 cm.

I got for

a) 110 cm and 90cm

b) 0.36s

c)0.72s

d)

99-100=10cos500t

500t=95.74

500t=264.3

t=0.191s

t=0.528

the proportion of time where the depth is less than 99 = 0.528-0.191 =0.338

But correct ans for this part is 0.00213s

Any help would be appreciated..