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Algebra · expressing quantities

P6 · Algebra · PSLE Maths

Question

Siti bought 3 boxes of pens. Each box had k pens. She gave 5 pens to her classmates. (a) Express the number of pens Siti had left in terms of k. (b) When k = 12, how many pens did Siti have left?

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Hint

First draw the 3 boxes as 3 groups of k, then subtract the 5 pens given away.

Show the answer and step-by-step explanation

Answer: Pens left 3k-5 pens; Pens left when k = 12 31 pens

  1. Draw 3 boxes — Each box has k pens. Siti bought 3 boxes, so draw 3 equal blocks. Each block represents k pens.
  2. Write 3k — Adding 3 groups of k gives 3k. This is the total number of pens bought.
  3. Subtract given away — She gave away 5 pens, so subtract 5 from the total, 3k.
  4. Answer part (a) — So, the number of pens left is 3k - 5 pens.
  5. Substitute k = 12 — For part (b), k = 12. Replace k with 12: 3 × 12 - 5 = 31. So Siti had 31 pens left. 📌 Key idea: first write the total using the letter, then add or subtract according to the question. Finally, substitute the given value of the letter.

Method: If each box has k pens, 3 boxes have 3k pens. Since 5 pens are given away, subtract 5. To substitute a value of k, replace k with the number and calculate.

Practice: 3 similar questions

  1. Ali bought 4 packets of stickers. Each packet had n stickers. He gave 7 stickers to his brother. (a) Express the number of stickers Ali had left in terms of n. (b) When n = 15, how many stickers did Ali have left?
    Answer

    0 stickers, 53 stickers — Each packet has n stickers, so 4 packets have 4n stickers. After giving away 7 stickers, the number left is 4n - 7. When n = 15, 4 × 15 - 7 = 60 - 7 = 53.

  2. Mei Ling made 5 trays of cookies. Each tray had p cookies. She then received 8 more cookies from her mother. (a) Express the number of cookies Mei Ling has now in terms of p. (b) When p = 9, how many cookies does Mei Ling have now?
    Answer

    0 cookies, 53 cookies — Each tray has p cookies, so 5 trays have 5p cookies. She received 8 more, so she has 5p + 8 cookies now. When p = 9, 5 × 9 + 8 = 45 + 8 = 53.

  3. Raju bought 6 boxes of coloured pencils. Each box had x coloured pencils. He gave 4 coloured pencils to each of 3 friends and kept the rest. (a) Express the number of coloured pencils Raju kept in terms of x. (b) When x = 10, how many coloured pencils did Raju keep?
    Answer

    0 coloured pencils, 48 coloured pencils — Each box has x coloured pencils, so 6 boxes have 6x coloured pencils. He gave 4 to each of 3 friends, so he gave away 3 × 4 = 12 coloured pencils. The number kept is 6x - 12. When x = 10, 6 × 10 - 12 = 60 - 12 = 48.

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