Completely confused with the wording of the problem.

Initially Terry was planning to read 90 pages/ day. But, she only reads 75 pages / day for few days and in the final 6 days, she needs to read 690 pages.
Thus, if we give her 16 days,
90*16 = 1440 pages and 690 + 75(16-6) = 690 + 750 = 1440.
Thus, option B.

with x = number of days

with her initial plan, we conclude the book number of pages is 90x
with the revised plan, the book number of pages is 75(x-6) + 690

90x = 75(x-6) + 690

90x = 75 - 450 + 690

x = 16

Answer B

I am not getting the point!

90pg/1day but spent 75pg/day with a remaining 690 pgs on the last 6 days.

We know she would have finished the book on time if she had read 90 pages per day but at this lower rate she won't be able to finish it on time. How many pages short will she be?

690 - 75*6 = 690 - 225*2 = 690 - 450 = 240 pages short.

We know she would have finished had she been reading 90 pages each day. The difference between these rates in 15pg a day. Since she fell behind 15 pages every day, 240/15 should give us the correct answer. (8*3*5*2)/(3*5) = 8*2 = 16

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