Let the height of the building be h and the distance between the tower and building be d.
Tower height = 60 m
Angle from building's foot to tower's top = 60°
Angle from tower's foot to building's top = 30°
From the foot of the building, looking at the top of the tower:
tan(60°)=d60
3=d60
d=360
d=203 m
From the foot of the tower, looking at the top of the building:
tan(30°)=dh
31=dh
Substituting d=203:
31=203h
h=3203
h=20 m
The height of the building is 20 m.