Well to find this out you need to make an equation
s+7=j
So for example if Johnny's sister is 15 then Johnny would be 22 because
s(sister's age) +7(number of years difference between their ages)=22(Johnny's age)
It was caused by a mix of different scenarios. Rome was constantly attacked by Barbarian tribes; they were over-expanded, which made it really hard for them to keep their entire empire under control; they had a lot of economic problems and relied too much on slave labor; and they spent way too much money on their military, just to name a few factors.
