Question on answer 10*

I have a question for Ms Teo with regards to answer 10* that you posted yesterday..

In doing the model that representing the fraction (3 over 8), you used 3 boxes to represent Singaporeans and the remaining 5 boxes to represent Malaysians and Indonesians... and following that, shouldn't the next fraction (3 over 7) be reprsented by 3 and 7 boxes where 3 boxes represent Malaysians and the remaining 7 represent the Indonesians instead of slicing the previous 5 boxes into 10 smaller boxes ??

I am a little confused with that so please help me to understand that ... Thanks !

Nathaniel