draw 3 overlapping circles

they all 3 overlap in the middle with 10 who read all 3 reading materials

Let x = the number who read both newspapers & magazines

0__>__x__>__10, maximum value of x is 30

75-25= 50 x__<__ 50

45-15 = 30 x__<__30

x is less than or = to 30

x__>__0, 0__<__x__<__30

x could be any integer between 0 and 30

there are 100 people

100 = 75+45+35 - overlaps = 155-10-15-5-x = 125-x

125-x = 100

x = exactly 25

The only reason x might be another number between 0 and 30 is if there are some people who didn't read any books, magazines or newspapers.