How many window coverings are necessary to span 50 windows if each window covering is 15 windows lon...........

Explanation:

Given that, 
Number of windows = 50
Each window covering covers 15 windows
=> 50 windows requires 50/15 window coverings
= 50/15 = 3.333
Hence, more than 3 window coverings are required. In the options 4 is more than 3. 
Hence, 4 window coverings are required to cover 50 windows of each covering covers 15 windows.