3. Grandfather, father, and son together have 84 years, where the number of years of the father is the arithmetic mean of the number of years of the grandfather and the son. How old is the father? (Note: The arithmetic mean of two numbers is obtained by dividing the sum of those numbers by 2.)
3. $d$ - number of years of the grandfather

$o$ - number of years of the father

$s$ - number of years of the son

It holds that:

$o=(d+s): 2$, or $d+s=2 o$.

2 POINTS

Since $o+(d+s)=84$, it follows that

$o+2 o=84$

1 POINT

$3 o=84$

$1 \text{POINT}$
$o=84: 3$

$o=28$

The father is 28 years old.

1 POINT