Answer:
A. 15
Explanation:
The angles are vertical angles, therefore they are congruent. We can set the two measures equal to each other.
![6a+10=3a+55](https://img.qammunity.org/2021/formulas/mathematics/middle-school/o1d4oqz9nlbaqyqp651hdu05o73pq0g02v.png)
Now, solve for a. We must get a by itself on one side of the equation.
First, subtract 3a from both sides of the equation.
![6a-3a+10=3a-3a+55](https://img.qammunity.org/2021/formulas/mathematics/middle-school/7yuqmmv5lwd0g0tijvs9npgragrdrsdocu.png)
![(6a-3a)+10=55](https://img.qammunity.org/2021/formulas/mathematics/middle-school/1tammq1d077sb995qnl9uwsioazo10t3lc.png)
![3a+10=55](https://img.qammunity.org/2021/formulas/mathematics/middle-school/hiz0j4cgnkwyglz6exk1q4gagwonxk6nxx.png)
Next, subtraction 10 to both sides of the equation. We subtract because 10 is being added to 3a. The inverse of addition is subtraction.
![3a+10-10=55-10](https://img.qammunity.org/2021/formulas/mathematics/middle-school/yb2m6e0qcnzagvjdhd5is84bj0axh5bwl0.png)
![3a=55-10](https://img.qammunity.org/2021/formulas/mathematics/middle-school/6l1uxdrlvxayjh9xsa2bcoboxel5gtu7fw.png)
![3a=45](https://img.qammunity.org/2021/formulas/mathematics/middle-school/u58fs0hqwjyokhd4cgao3s4tq40hkz5fg2.png)
3 and a are being multiplied. The inverse of multiplication is division. Divide both sides of the equation by 3
![3a/3=45/3](https://img.qammunity.org/2021/formulas/mathematics/middle-school/5rjeiosrio3icph1ksbrfq9fn6ncec6zf3.png)
![a=45/3](https://img.qammunity.org/2021/formulas/mathematics/middle-school/h32crtxncb21xuch6zw9rjpbvn9v3tbzok.png)
![a=15](https://img.qammunity.org/2021/formulas/mathematics/middle-school/6v5pfb3ha63be7ybgzwajfhhyfcjvokw5m.png)
The value of a is 15 and choice A is correct.