Solving by simultaneous equations.
Tex + Oak = 14
Tex - Oak = 6
Since both of the above statements are true, then adding them up still gives us a true statement. (the Texes add up, the Oaks cancel out)
2 x Tex = 20
and dividing both sides by 2
Tex = 10
Substituting Tex in either of the original two equations and solving for Oak gives
Oak = 4.
Checking our work by substituting into the original equations:
10 + 4 = 14
or, alternatively,
10 - 4 = 6