You can read posting Sarin and his coin bank on the story of coin bank.
Read also posting on Sarin and the million numbers to understand concept of dollars and cents.
Lower primary school mathematics question LPQ48
Sarin had a total of twenty $10-notes and $5-notes in his coin bank. The total value of all the notes was $120. How many $10-notes did he have?
A set of 1 $10-note and 1 $5-note give value of = 10 + 5 = $15
If there are 10 sets, the value = 10 × 15 = $150
It is more than $120 by $30
To reduce $10-notes and increase $5
We needs to reduce 30 ÷ 5 = 6 $10-notes to get to $120
The number of $10-notes = 10 – 6 = 4
The number of $5-notes = 10 + 6 = 16
Use guess and check method
Start the guess from 10 $10-notes and 10 $5-notes
Remember the total number of notes = 20 (1st check)
Then base on total value coputed to decide increase or reduce the number of notes
Sarin has 4 ten dollar notes in his coin bank.