Question

Amy, Siti and Tina have a total of 630 story books. Tina has 50 books more than Amy, Amy has 70 books more than Siti. How many story books does Tina have?

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Answer 1

Siti's books = x
Amy's books = x+70
Tina's books = (x+70)+50=x+120

All books = x + (x+70) + (x+120)
And all books = 630

It means equation:
x + x + 70 + x + 120 = 630
3x + 190 = 630
subtract 190 from both sides: -190 -190
3x = 440
divide both sides by 3: :3 :3
x = 146 2/3

Tina's books: x+120 = 146 2/3 +120=266 2/3

It means Tina has 266 or 267 story books (because book in fractions doesn't count for a book)
Or you wrote wrong numbers :)

Answer 2

Let us assume the number of story books that Siti has = x
Then
The number of story books that Amy has = x + 70
The number of story books that Tina has = (x + 70) + 50
= x + 120
Then
x + x + 120 + x + 70 = 630
3x + 190 = 630
3x = 630 - 190
3x = 440
x = 440/3
Then
The number of books that Tina has = (440/3) + 120
= (440 + 360)/3
= 800/3
= 266.66
So Tina has 267 books as the number of books cannot be represented by a fraction.