Let L be the set and the number of those students who has a laptop     (L = 800);
    T be the set and the number of those students who has a tablet     (T = 300).


Do not worry that I denoted by the same symbol the set and the number: I made it for simplicity, 
and you always can distinct from the context what I am talking about.


Let LT be the intersection of the sets L and T and the number of elements in this set at the same time.


Notice that LT is the set of students who has both devices, and their number is 250, according to the condition.


Consider the set L U T.  It is the set of students who has at least one of the two devices.


There is a simple and elementary formula for the number of elements n(L U T) in the set L U T.   This formula is

n(L U T) = L + T - LT.     (1)


Using it, you can easily calculate n(L U T) in your case. It is

n(L U T) = 800 + 300 - 250 = 850.


Thus the number of students who has  no any of the two devices is 1000-850 = 150.


Now you have everything to answer your questions.


1)  -What is the probability that a randomly selected student has neither device?

     It is   =  = 0.15.


2)  -What is the probability that a randomly selected student has a laptop, given that he/she has a tablet?

     It is   = .


3)  -Let event A be the selected student having a laptop, and event B be the selected student having a tablet. 
     Are A and B independent events? Why or why not?

     These events are not independent.

     Why?  - Because the sets L and T nave non-empty intersection.

    To find the number of elements in the UNION of two subsets, we must add the numbers of elements in subsets 
    and then subtract the number of elements in the intersection that we counted twice.