Parker is 6 years old
Let q, p, k represent the ages of Quinn, Parker, and Karl, respectively.
... q = 2k . . . . . . Quinn is twice as old as Karl
... q = p+4 . . . . . Quinn is 4 years older than parker
... q + p + k = 21 . . . . the sum of their ages is 21
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We can use the first two equations to find Karl's age in terms of Parker's.
... p+4 = q = 2k
... (p+4)/2 = k
Now, we can substitute for q and k in the last equation.
... (p+4) +p +(p+4)/2 = 21
... 5/2p +6 = 21 . . . . . . collect terms
... p = (2/5)(21 -6) . . . . subtract 6, multiply by 2/5
... p = 6 . . . . . . Parker is 6 years old
