Answer:
57.7 m
Step-by-step explanation:
Since the distance between Isabella, the building and the angle form a right angled triangle with the height of the building = h = 50 m as the opposite side and x = distance between Isabella and the building = adjacent side and Ф = angle of elevation of the top of the building from Isabella = 30°.
So, from trigonometric ratios,
tanФ = h/x
and x = h/tanФ 
= 50 m/tan30° 
= 50/0.5774 
= 86.6 m
Also, the distance between Alice, the building and the angle form a right angled triangle with the height of the building = h = 50 m as the opposite side and x' = distance between Alice and the building = adjacent side and Ф' = angle of elevation of the top of the building from Alice = 60°.
So, from trigonometric ratios,
tanФ' = h/x'
and x' = h/tanФ' 
= 50 m/tan60° 
= 50/1.7321
= 28.87 m
So, the distance between Isabella and Alice, x" = x - x' 
= 86.6 m - 28.87 m 
= 57.73 m 
≅ 57.7 m