Chinese Math Olympiad (Primary) Trick vs Singapore Modeling Math

Both Methods (I) & (II) below does not use Algebra, which is not taught to primary school kids until secondary school (above 12 or 13 years old).

(I) Ancient Chinese Arithmetic Method:

例2：The sister is 13 years old this year, while the brother is 9 years old.
When the total of their ages is 40, how old will they be?

【这位妈妈太绝了！竟把小学6年奥数化为13句口诀】

Trick: Age difference does not change over time, they + or – in tandem. When 2 ages change, the multiple between them also changes.

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(II) Singapore Modeling Math:

［例2］

Compare the Ancient Chinese Arithmetic (I) & Singapore Modeling Math (II):

• (II) is better for young kids to understand visually the logic behind,
• (I) is more a memorised trick but poor math education pedagogy. In [例 2] it is very hard for kids to understand why the 2 steps (40+4) & (40 – 4), although they lead to…