In a bag of marbles, the ratio of red to blue marbles is 4 : 5. If there are 81 marbles in total, how many red marbles are there?

easy 1 mark Class 6 · Mathematics · Ratio and Proportion

Answer

The correct answer is 36 — The number of red marbles is $4 x = 4 \times 9 = 36$ .

Step-by-step solution

Let the number of red marbles be $4 x$ and the number of blue marbles be $5 x$ because the ratio is $4 : 5$ .
The total number of marbles is $4 x + 5 x = 81$ .
Solve $9 x = 81$ to get $x = 9$ .
The number of red marbles is $4 x = 4 \times 9 = 36$ .

Common mistake: Adding 4 and 5 instead of treating them as parts of the same whole.

Concept tested: Finding part from a given ratio