You can read posting Sarin and his coin bank on the story of coin bank. Read also postings on Sarin and the million numbers to understand concept of dollars and cents and Sarin learns the concept of Fraction (mathematics concept) on concept of fraction.
Upper primary school mathematics question UPQ25
Sarin has 279 coins in his coin bank. 3/5 of them are 20-cent coins and the rest are 50-cent coins. He spent 1/3 of the 50-cent coins on a book. How much did the book cost?
Cost of a book = 279 × 2/5 × 1/3 × 50
= 1860cents = $18.60
The fraction model
1/3 of the 50-cent coins used on a book
The 20-cent coins is 3/5 of the fraction of the total coins
The 20-cent coins take 3 parts and 50-cent coins take 2 parts
We can divide the 2 parts to 6 sub-parts and 2 sub-parts as the cost of the book
From the model
The total number of coins = 279
15 units = 279
1 unit = 18.6
The cost of a book = 2 × 18.6 ×0.50 = $18.60