let J, S, and F = the current age of Jim, Fred, and Sam, respectively and create the equations:
J = 2F
And
S = F - 2
S + 2 = F
And
J - 4 = 4(S - 4)
J - 4 = 4S - 16
J = 4S - 12
Since J = 2F we have:
2F = 4S - 12
F = 2S - 6
Since S + 2 = F, we have:
S + 2 = 2S - 6
8 = S, so J = 4 x 8 - 12 = 20.
Answer: 20
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