Eighteen years ago Ivan was three times as old as his son but he is only twice as old now. How old is Ivan?

(Not a trick question.)

72

Step-by-step explanation:

Let Ivan's current age be g and his son's current age by s.

Then g - 18 = 3 (s - 18), and

g = 2s

Substituting 2s for g in the first equation, we get:

2s - 18 = 3s - 54

Grouping like terms, we get:

36 = s.

Thus, Ivan's son's age now is 36 years; Ivan is twice that, or 72 years.