Let's solve question 51:

Question: The sum of 15% of a positive number and 20% of the same number is 126. What is one-third of that number?

Let the positive number be $x$.

The sum of 15% of $x$ and 20% of $x$ is given as 126. This can be written as: $0.15x + 0.20x = 126$

Simplify the left side: $0.35x = 126$

Now, solve for $x$: $x = \frac{126}{0.35} = 360$

So, the number is 360.

The question asks for one-third of that number: $\frac{360}{3} = 120$

Answer: The correct answer is (c) 120.

Let me know if you'd like more details or have any other questions!

Here are 5 related questions:

1. What is 10% of 360?
2. If 25% of a number is 90, what is the number?
3. How do you find a percentage of a number?
4. What is one-fifth of 200?
5. If the sum of 30% and 40% of a number is 210, what is the number?

Tip: When working with percentages, converting them to decimals (e.g., 15% = 0.15) helps simplify the equation and calculations.