In 6 months Bella and Bob will be celebrating their 4th anniversary.  How many months ago did they celebrate their 2nd anniversary?
First, we find their current point in time be realizing that 4 years is 4 sets of 12 months, meaning at that anniversary they will have been together 4*12=<<4*12=48>>48 months
Then, we subtract their 6 month difference, finding they are at 48-6=<<48-6=42>>42 months into their relationship.
Since 2 years would be 2*12=<<2*12=24>>24 months into the relationship.
We take the larger number, 42, and subtract the smaller number, 24 to find the difference in months, meaning it is 42-24= <<42-24=18>>18 months
#### 18