Val, all my kids and I learn fraction the way Professor Wu recommended. The LCM approach was taught after the foundation was already laid and understanding secured. I was taught by an ancient (expert) 5th grade teacher and a brilliant 6th grade teacher (engineering degree/experience) who were masters at in-class differentiation.
My children went to a school on the other side of the country from where I grew up and were seven years apart. Basic fractions were introduced in second grade math. Fraction manipulations were taught step by step in 3rd grade GT math (3rd and 4th grade math combined) along the lines of Professor Wu's approach. Then more complicated fraction manipulations like division were presented in 4th grade GT math (5th grade math). By 5th grade GT math (6th grade math), the students were expected to use the LCM approach and cross reduction efficiently. Based on the discussions and assessments prior to DS' acceleration into Pre-algebra at the beginning of 4th grade, our district predicates acceleration into Pre-algebra on mastery of fractions.
