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?
pl show working out

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.

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 :)