A 1.5 m tall boy

Let AB be the boy, and CD be the building and angles becomes 60°, as the boy reaches E.

The angle of elevation in first case = 30°

And

The angle of elevation, in the second case = 60°.

Difference between bases in the first case and second case will give the distance covered by the boy.

1st case:
here p = height of the building - the height of the boy  
          = 30 - 1.5
          = 28.5 m

b = 28.5√3 

2^nd case:

Hence, the distance covered by the boy:

= 28.5

= 19√3 m