An algebra problem by Mahmdullah Khaliq

When you are $10$ years old your sister is $10 \times$ $\dfrac{1}{2}$ or $0.5 = 5$

Now we know that your sister is $5$ years younger then you.

Therefore $90$ years later, when you are $100$ years old your sister will be $(100 - 5)$ $= \boxed{95}$

the difference will remain the same as ..5 years

When you are $10$ years old, your sister is $5$ years old. Then, $90$ years later, when you are $100$ years old, your sister must be $\boxed{95}$ years old.

The answer is $100 - 5 = \color{#69047E}{\boxed{95}}$ .