Sara is 25 years older than teresa. in 4 years she will be twice as old as teresa. how old are they now?

The key is to write down the two equations that are implied here:

First, Sara (S) is 25 years older than Teresa (T) : S = 25 + T

Second, in 4 years S will be twice as old as T: S + 4 = 2 (T+4).

The last equation can be solved for S, which looks, after distributing, like:

S = 2T + 4

You can use substitution of this equation into the other to solve for T and then use that value to solve for the value of S.