Question 858224
height of tower = 68m

angle of depression = 47 deg

angle of depression after 10 minutes  = 15 deg

10 min = 600 seconds

Let the initial ground distance from tower to yacht x m

Tan 47 = 68/x
x= 68/tan 47

Let the speed of the yacht = y

distance traveled in 600 seconds = 600y

Tan 15 = 68/(x+600y) 

x+600y = 68/tan 15

x= (68/tan 15)-600y

68/tan47 = (68/tan 15)-600y

600y = (68/tan 15)- (68/tan 47)

600y=253.8-63.4

y=0.32 m/s------ speed of the yacht