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.
Upper primary school mathematics question UPQ79
Sarin and Hairu saved a sum of money. Sarin’s share was $180 less than Hairu’s share. After Hairu gave 1/7 of his share to Sarin, Sarin had $20 more than Hairu. How much did Sarin have at first?
Hairu had $180 more than Sarin at first, if he gave $90 to Sarin, they will have the same amount.
For Sarin to have $20 more, he need to receive $90 + $10 = $100 from Hairu.
We can draw a math model based on the $180 more that Hairu had,
Since, Hairu gave 1/7 of his money to Sarin is $100
Hairu had 100 × 7 = $700 at first
Sarin had at first = 700 – 180 = $520