13 pens + 8 books = 21 pens + 6 books
Move all the books to the left and pens to the right:
8 books - 6 books = 21 pens - 13 pens
Combine like terms:
2 books = 8 pens
Divide both sides by 2:
1 book = 4 pens
Given:
The price of 1 book is the same as 4 pens.
Find all pens:
13 pens + 8 books = 13 pens + 8(4) pens = 45 pens.
Answer: (a) He can buy 45 pens.
Question b:
Define x:
Let x be the number of pens.
pen = x
book = 2(4x) = 8x
Find x:
9x = 45
Divide both sides by 9:
x = 15
Pens = 15
Books = 30
Answer (b) He can buy 15 pens and 30 books.