In a school . 35 students like reading , 50 student like jogging , and 15 students like both . How many person like either reading or sports ?

The ans is 35+ 50 -15 = 70 ( Ans given)

but , i think the ans should be 20+ 35 = 55 only

Which is correct ? If my ans is wrong , in what circumstances , the ans would be 20+ 35 = 55 ??