1. What is an abacus? 
2. How does an abacus help in learning subtraction? 
3. How do you subtract using the hundred's column on an abacus? 
4. Can an abacus be used for complex subtraction calculations? 
5. Are there any online resources or apps available for learning abacus subtraction? 
Abacus Lesson 18 // SubtractionComplementary Numbers Respect to 5 ONE'S Column Video  10:59 min 
Abacus Lesson 19 // SubtractionComplementary Numbers Respect to 5 TEN'S Column Video  07:20 min 
Abacus Lesson 20 // SubtractionComplementary Numbers Respect to 5 HUNDRED'S Column Video  04:36 min 
Abacus Lesson 21 // Intro. to Complementary Numbers with Respect to 10 Video  07:07 min 
Abacus Lesson 22 // Addition Complementary Numbers Respect to 10 ONE'S Column Video  08:05 min 

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