Asked by Matew

three bookshelves contain a total of 35 books. The first and second shelves contain a total of 20 books. the second and third shelves contain a total of 28 books. how many books are on the second shelf then?
Answered by Reiny
first one --x
2nd one -- y
3rd one -- 35-x-y

x+y = 20
y + (35-x-y) = 28 --- > x = 7

(then 7+y =20 ---> y = 13
35-x-y = 35-7-13 = 15

first shelf has 7
2nd shelf has 13
third has 15

check:
1st plus 2nd = 7+13 = 20 , check!
2nd plus 3rd = 13+15 = 28, check!
all is good
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