John invests $\text{Rs. }18,000$ in stocks and Sarah invests $\text{Rs. }1,00,000$ in stocks. What is the ratio of John's investment to Sarah's investment?

To find the required ratio, we divide John's investment by Sarah's investment:
$$\text{Ratio}=\frac{\text{John’s Investment}}{\text{Sarah’s Investment}}$$

$$\text{Ratio}=\frac{18,000}{1,00,000}$$

Simplifying by cancelling the common zeroes:

$$\text{Ratio}=\frac{18}{100}$$

Dividing both the numerator and denominator by their greatest common divisor, which is $2$:

$$\text{Ratio}=\frac{18÷2}{100÷2}=\frac{9}{50}=9:50$$