Let x be Dr. Garcia's age, and y his sons age.

1. Dr. Garcia is twice as old as his son

x = 2 y

2. Twenty years ago, he was 4 times as old as his son was then

x - 20 = 4 (y - 20)

Substitute x in 2 and solve for y:

2 y - 20 = 4 (y - 20)

2 y - 20 = 4 y - 80

- 20 = 2 y - 80

60 = 2 y

y = 30

Put y back into 1:

x = 2 (30)

x = 60

So Dr. Garcia's age is 60, and his sons age is 30.