Ravi had a box of building blocks. 38 of the blocks were red and the rest were blue. Each red block had a mass of 20 g and each blue block had a mass of 12 g. The total mass of all the blocks was 1.92 kg.
- How many blue blocks were there?
- Ravi then gave away 25% of the blue blocks. He kept all the red blocks. What was the total mass of the blocks he had left? Give your answer in kilograms.
Need a hint?
Change 1.92 kg to grams first. Then find the mass of one group of 8 blocks (3 red and 5 blue).
Worked answer
- (a) Think of the blocks in groups of 8: 3 red and 5 blue.
- Mass of 3 red blocks = 3 × 20 = 60 g. Mass of 5 blue blocks = 5 × 12 = 60 g.
- One group of 8 blocks has mass 60 + 60 = 120 g.
- 1.92 kg = 1920 g, so the number of groups = 1920 ÷ 120 = 16.
- Blue blocks = 16 × 5 = 80.
- (b) Red blocks = 16 × 3 = 48.
- 25% of 80 = 20, so blue blocks left = 80 − 20 = 60.
- Mass of red blocks = 48 × 20 = 960 g. Mass of blue blocks = 60 × 12 = 720 g.
- Total mass left = 960 + 720 = 1680 g = 1.68 kg.
Answer: 1.68 kg
The mistake this question tests
Habit
Converting units (kg, ℓ, m, cm³)
Many pupils use 1.92 kg as 1920 g but then divide by 20 or 12 alone, or forget to change grams to kilograms at the end. They also forget that the red blocks stay the same when only the blue blocks are given away.
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