The lowest common multiple of 3,4 and 6 is 12. Therefore if you multiply the ratio by 12 you get 12/3 : 12/4 : 12/6 which is equal to 4:3:2.
When you add 4,3 and 2 the answer is 9 therefore the £90,000 is split into 9 parts. £90,000 divided by 9 = £10,000 which means each part is £10,000.
The wife receives 4 parts so she receives 4 x £10,000 = £40,000
The son receives 3 parts so he receives £30,000
The daughter receives 2 parts so she receives £20,000