A little help please Mathematicians :D

Right so I have exams coming up in a week and I can't seem to work my head around this type of problem, eg:

Three generations of the Smith family are looking at some photos. Jess is the youngest. Her mother Kate is twice as old as Jess, while Mary her grandmother, is twice as old as Kate's age minus 19.

I think this is a simultaneous equation but I can't remember how to do them :(

Can someone please show me the solving process, it'd be a great help :D

If the ages of all three total 100, how old is Mary?

Nice ambiguity.
Is "twice as old as Kate's age minus 19" 2*Kate-19 or 2*(Kate-19)?

I'm assuming the former.

K = 2J
M = 2K-19
K+J+M = 100

M = 4J-19
K+J+M = J + 2J + 4J - 19 = 7J - 19 = 100
J = (100+19)/7 = 17 years old
K = 2J = 2*17 = 34 years old
M = 4J-19 = 4*17-19 = 49 years old

What is this middle school bullshit?

And why the fuck do they give you math problems about fucked up families? The grandmother apparently gave birth to the mother at age 15, and the mother gave birth to the daughter at age 17.