Direct proportion: Class 8 practice
Two quantities are in direct proportion when they increase or decrease in the same ratio: x/y stays constant. Read the method, follow the three worked examples, then try the six questions.
What you should know first
- Ratio
- Unitary method
The idea behind direct proportion
Two quantities are in direct proportion when they increase or decrease in the same ratio: x/y stays constant. If x₁/y₁ = x₂/y₂ you can find the missing value.
Step-by-step method
- Check both quantities change in the same direction.
- Write x₁/y₁ = x₂/y₂.
- Cross-multiply and solve.
Worked example 1 (Basic)
9 litres of petrol give 54 km. How many km for 18 litres of petrol?
- 9/54 = 18/y
- y = 54 × 18 ÷ 9 = 108
- Answer: 108 km
Worked example 2 (Standard)
9 litres of petrol give 72 km. How many km for 11 litres of petrol?
- 9/72 = 11/y
- y = 72 × 11 ÷ 9 = 88
- Answer: 88 km
Worked example 3 (Challenge)
5 hours give 60 pages printed. How many pages printed for 16 hours?
- 5/60 = 16/y
- y = 60 × 16 ÷ 5 = 192
- Answer: 192 pages printed
Common mistakes and misconceptions
- Using inverse proportion by mistake
- Mixing up which value goes where
Practice questions
- (Basic) 2 hours give 6 pages printed. How many pages printed for 20 hours?
- (Basic) 2 kg of flour give 10 loaves. How many loaves for 19 kg of flour?
- (Standard) 3 kg of flour give 21 loaves. How many loaves for 13 kg of flour?
- (Standard) 2 litres of petrol give 12 km. How many km for 4 litres of petrol?
- (Challenge) 5 litres of petrol give 15 km. How many km for 13 litres of petrol?
- (Challenge) 7 kg of flour give 14 loaves. How many loaves for 19 kg of flour?
Answer key with full working
- Q1: 2/6 = 20/y → y = 6 × 20 ÷ 2 = 60 → Answer: 60 pages printed
- Q2: 2/10 = 19/y → y = 10 × 19 ÷ 2 = 95 → Answer: 95 loaves
- Q3: 3/21 = 13/y → y = 21 × 13 ÷ 3 = 91 → Answer: 91 loaves
- Q4: 2/12 = 4/y → y = 12 × 4 ÷ 2 = 24 → Answer: 24 km
- Q5: 5/15 = 13/y → y = 15 × 13 ÷ 5 = 39 → Answer: 39 km
- Q6: 7/14 = 19/y → y = 14 × 19 ÷ 7 = 38 → Answer: 38 loaves
Syllabus note
This skill is listed under "Direct and Inverse Proportions" for Class 8 in the NCERT/CBSE books. ICSE, IGCSE, IB and state boards may teach it in a different chapter or year.
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.
Related practice and notes
Frequently asked questions
Which chapter is direct proportion in?
"Direct and Inverse Proportions", Class 8 Maths (NCERT/CBSE). Other boards may place it differently.
How were the answers checked?
Answers on this page are generated and automatically re-checked by a second, independent calculation. They have not been reviewed by a teacher — if you spot a problem, tell us on WhatsApp.