Answer :
500 (the current collection) + 10 (the weekly addition) x 20 (weeks to add)
500 + 10*20 (Multiplication comes before addition)
500+200
700
500 + 10*20 (Multiplication comes before addition)
500+200
700
500 total righ now
each week he adds 10
after 20 weeks he will have x books
20 weeks=10 times 20 books added=200 books added
500+200=700
answer is 700
each week he adds 10
after 20 weeks he will have x books
20 weeks=10 times 20 books added=200 books added
500+200=700
answer is 700