Given that each covering is 15 windows long you need to calculate how many times is 15 in 50, to know how many coverings are necessary to cover the 50 windows.
The operation is a simple division: 50 ÷ 15 gives the number of times that 15 is contained in 50.
The result is 50÷15 = 3.33.
So, you need more than 3 coverings, and so take the next whole number, i.e. 4.
Answer: 4