Bridget is 10 years younger than Dana.
Marsha is 5 years younger than Bridget.
Dana is twice at old as Marsha.

So we have 3 variables (the ages of Dana, Marsha, and Bridget), and 3 equations...so we can get an answer.

D is age of Dana
M is age of Marsha
B is age of Bridget

First sentence: Bridget is 10 years younger than Dana.
Equation: B = D - 10

Second sentence: Marsha is 5 years younger than Bridget.
Equation: M = B - 5

Third Sentence: Dana is twice at old as Marsha.
Equation: D = 2M

Ok...B = D - 10.  Therefore, wherever there is a B, we can substitute "D - 10" for it, because they are equal.

Now take the second equation: M = B - 5
M = B - 5
M = (D - 10) - 5
M = D - 10 - 5
M = D - 15

Now since M = D - 15, we can substitute "D - 15" for any M we see.

Third equation: D = 2M
D = 2(D - 15)
D = 2D - 30
-D = -30
D = 30

Now to get Bridget's age, just subtract 10, because we know that already.
If you want Marsha's age, just subtract 5 from Bridget's age, or 15 from Dana's (or divide Dana's by 2)

I hope this helps!