In this question we have given
mass of Ebony=75Kg
His sister mass=10kg
here total mass=mass of Ebony+his sister mass
m=75+10
m=85kg
radius of circular path=11m
angular momentum=14020kgm^2/s
we know that,
angular momentum=interia*angular speed
angular momentum=I*w...........(1)
here, interia I=mr^2
and w=v/r (here v is the linear velocity and r is radius)
put values of I and w in equation (1)
angular momentum=mr^2*v/r
angular momentum = mvr
14020 = (75+10) x v x 11
v = 14020/(85x11) = 14.99m/s