The equation describes the problem C.
Why?
If we need to find which of the problem situation is represented by the equation, we need to solve the equation and check which makes sense.
Checking the problem C, we can see that it's the correct option.
We have the following equation:
![(1)/(2)x+5=20\\](https://img.qammunity.org/2020/formulas/mathematics/middle-school/ed6j36kksynk1knq1vtp0p0rg9f0lp0toy.png)
Solving, we have:
![(1)/(2)x=20-5\\\\x=15*2=30](https://img.qammunity.org/2020/formulas/mathematics/middle-school/61sz03ijdd8a9uggaumrbqiwogf17t08l9.png)
We can see that her sister is 30 years old.
If we substitute the value into the equation, we can see that the equation is satisfied, so, it's the correct option.
The correct option is C.
Have a nice day!