STEP 1: calculate the amount of pages not read per book and put them into a fraction ( e.g for the first one 30/70 pages not read)
STEP2: turn all of the fractions you have just made into percentages ( divide the numerator by the denominator then, times by 100)
STEP 3: add all of the percentages up and put the sum of them all over 300 ( e.g each percentage added up / 300 )
STEP 4: turn that fraction you just made back into a percentage then that should be your answer!!!
If you need any more help just ask me!