Explanation:

Let the marks obtained per question be a random variable X its probability distribution  is

X          1          -0.25

P(x)     ¼         ¾

Expected marks per question =  =∑X p(x)

=1 × 1/4  +(-0.25)  × ¾

=1/4 -3/16

=1/16 marks

Total marks expected for 150 questions =1/16× 150

=75/8 marks per student

Total expected marks of 1000 students =75/8 ×1000

=9375  marks