P5 fractions (fr: hanyin)

A box contains 20-cent coins n 50-cent coins.1/4 of the number of 20-cent coins is equal to 3/5 of the number of 50-cents coins.there is a total of 102 coins in the box.find the total value me all the coins in the box.
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My solution

http://www.showme.com/sh/?h=MsH2Crw
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Sec 1: whole numbers: Composite numbers

My Solution:

Part (c)

Factors of 36: 1, 2, 3, 4, 6, 9, 12, 18 and 36

Factors of 54: 1, 2, 3, 6, 9, 18, 27 and 54

Since 1, 2, 3 are prime numbers, the composite numbers which are factors of both 36 and 54 are only:

6, 9 and 18

Part (d)

LCM of 24 and 36 is 72.   (Hope you know how to get this ;p )

The smallest number that we are looking for that when divided by either 24 or 36 and yield a remainder of 11 would be:  

72 + 11 = 83

Sec 1: whole numbers LCM, HCF

My solution:

Given that one of the number is 56.

With the LCM being 504,

      504 / 56 = 9

The other number must be a multiple of 9.

With the HCF of the 2 original numbers being 8,

The smallest possible value of the 2nd number has to be 8x9 = 72