He’s right For the answer
They are 6, 8, 10, 12 years old.
all of those added is 36 and two-year intervals translates to 2 years apart per cousin.
You said x + 2/5 = 12
It might be easier to handle if we use the decimal for 2/5.
x + 0.4 = 12
Subtract 0.4 from each side: x = 11.6
That's all there is to it. I'm sorry I can't make it
more complicated for you.