Question: 15. Class five pupil asked his friend that which of the following is a valid number in base 2? A. 1012 B. 1201 C. 1101 D. 2010 16. Madam Priscilla Yeboah tells one of his pupils to come and demonstrates how to convert the decimal number 6 to binary (base 2). Which one below is CORRECT option? A. 1010 B. 1001 C. 110 D. 111 17. Ama asked Kofi that which of the following is not a valid number in base 5? A. 430 B. 234 C. 528 D. 120 18. Class four pupils expressed the number 234 in expanded form as .... A. 200 + 3 + 4 B. 20 + 30 + 4 C. 200 + 300 + 4 D. 200 + 30 + 4 19. The class teacher asked the pupils in the class what is the value of 6 in base 2? A. 100 B. 110 C. 101 D. 111 20. Which of the following teaching-learning materials is the most appropriate for teaching addition of two-digit whole numbers with re-grouping? A. Abacus B. Bottle tops C. Cuisenaire rods D. Number line

1. **Problem Statement:** Identify valid numbers in base 2 and base 5, convert decimal to binary, express numbers in expanded form, and select appropriate teaching materials. 2. **Base 2 Validity:** A valid base 2 number contains only digits 0 and 1. 3. **Check options for question 15:** - A. 1012 (contains digit 2, invalid) - B. 1201 (contains digits 2 and beyond, invalid) - C. 1101 (only 0s and 1s, valid) - D. 2010 (contains digit 2, invalid) **Answer:** C. 1101 4. **Convert decimal 6 to binary:** - Divide 6 by 2 repeatedly: $$6 \div 2 = 3$$ remainder 0 $$3 \div 2 = 1$$ remainder 1 $$1 \div 2 = 0$$ remainder 1 - Write remainders from bottom to top: 110 **Answer:** C. 110 5. **Base 5 Validity:** Valid digits are 0,1,2,3,4. 6. **Check options for question 17:** - A. 430 (digits 4,3,0 valid) - B. 234 (digits 2,3,4 valid) - C. 528 (digit 5 invalid in base 5) - D. 120 (digits 1,2,0 valid) **Answer:** C. 528 7. **Expanded form of 234:** - 234 means 2 hundreds + 3 tens + 4 ones - So expanded form is $$200 + 30 + 4$$ **Answer:** D. 200 + 30 + 4 8. **Value of 6 in base 2:** - From step 4, 6 in binary is 110 **Answer:** B. 110 9. **Teaching-learning material for addition with regrouping:** - Abacus is most appropriate for teaching addition with regrouping as it visually represents place values and regrouping. **Answer:** A. Abacus