The sum of the ages of a wood plaque and a bronze plaque is 20 years. Four years ago, the bronze plaque was one-half the age of the wood plaque. Find the present age of each plaque.

let age of wood plaque = x and age of the bronze one be y 
Then we have the system of equations:-

x + y = 20 ..................................(1)
y - 4 = 0.5(x - 4)
y - 0.5x = 2.................................(2)
Subtract (2) from (1):-
1.5x = 18
x = 12

from equation (1):-
12 + y = 20
y = 8

ANSWER: bronze plaque is 8 years old and wood plaque is 12 years old.