Math problem

I tried to teach Keke addition.

"2 8 16" I taught Keke in Chinese the keys.  "It means 8 plus 8 is 16."

Later, I taught Keke how to figure out 8+7,  'since 8+8=16, then 7 is one less than 8,
so 8+7=15.'

Keke understood that. We did not want her to count her fingers everytime to get the answer.