The oldest son spent $10 on candy. 40 + 25 = 65 + 10 = 75 then the other 10 was spent on candy by the oldest son
Given :

We know the identity
We use this property to simplify the left hand side
So 

we know ,
when
then

For 

Add
on both sides

Finally we add 2npi for general solution
So options C and D are correct
I’m sure your answer is D. 180 - 125 will get you 55.!
Have a nice day!