Let "x" be the age of Markus's grandson.
If Markus's son is twice the age of Markus's grandson, then Markus's son is 2*x.
If Markus is twice the age of his son, then Markus is 2*2*x.
Therefore, if the sum of the ages of Markus, his son, and his grandson is 140 years, then x+(2*x)+(2*2*x)=140 years.
Simplifying the equation, we see that 7*x=140 years.
Therefore, the age of Markus's grandson is x=20 years old.
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