Markus is twice the age of his son, and Markus's son is twice the age of Markus's grandson.  If the sum of the ages of Markus, his son, and his grandson is 140 years, then how many years old is Markus's grandson?
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.
#### 20