The blog postings are about the Singapore Math. The readers can learn from the postings about Solving Singapore Primary School Mathematics. The blog presenting the Math Concept, Math Questions with solutions that teaches in Singapore Primary Schools. You or the kids can aquire the skills of dealing with Math Model, Problem Sum start from Lower Primary School to Upper Primary School level after read the postings. This posting is an upper primary school math question on Ratio and Problem Solving.

You can read posting **Sarin and his coin bank** on the story of coin bank.

You should read the posting on **Sarin learns concept of ratio in school** to understand mathematics concept of Ratio.

Read also postings on **Sarin and the million numbers** to understand concept of dollars and cents.

*Challenge yourself to solve the Question before look out for the given solution‼!*

**Upper primary school mathematics question UPQ291**

Sarin had some 20-cent coins and $1 coins in the ratio of 4 : 1 in his coin bank. He took out three $1 coins and exchanged them for 20cent coins. The ratio of the number of 20cent coins to the number of $1 coins became 13 : 1. Find the sum of money Sarin had in his coin bank.

Solution:

Three number of $1 coins can be exchange to 3/0.2 = 15 number of 20-cent coins

Draw the model based on the ratio of the number of coins

Since 3 of $1 coin taken out and exchanged to 15 of 20-cent coins and the ration of $1 to 20-cent = 13 : 1

The old 1 unit is equal to a new 1 unit plus 3

Re-draw the model

Per the model

9 units = 3 × 4 + 15 = 27

1 unit = 3

The amount of money in his coin bank = 13 × 3 × 0.2 + 3 = $10.80

Alternative Solution:

The initial ratio, 20-cent : $1 = 4 : 1

Exchanged 3 $1 to 20-cents of 3/0.2 = 15

Add 15 of 20-cent coins and substract 3 1$ coins from the initial ratio

The final ratio, 20-cent : $1 = 13 : 1 = (4U + 15) : (1U – 3)

4U + 15 = 13 × (1U – 3)

4U + 15 = 13U – 39

9U = 54

1U = 6

Sum of money in his coin bank = 4 × 6 × 0.2 + 1 × 6 = $10.80