It helps to develop a sense of abstraction right from the start. You can do this by writing definitions. Make simpler things represent more complicated things.
Mother's Age - for example, might be just M.
Her Son's Age might be just S.
Her Daughter's Age might be just D.
The abstraction, of course, is to think of M, S, and D as a number that you actually know - or maybe have a little faith that you will know it. This gives you permission to deal with it.
How old is Mom? M - of course. What is the current value of M? This is where the faith comes in. The definition is good enough for now.
After definitions, one must learn to translate.
"The combined ages of a mother, her son, and her daughter"
What does that mean? From your definitions, you need to see that this is M + S + D. We simply combined all the ages with addition! The "addition" may not have been obvious, but we know it's a linear system, so it's less likely that it will be more complicated than that.
Where does that leave us?
