<human>:  Comet Halley orbits the sun every 75 years. Bill's dad saw the Comet when he was 30 years old. Bill saw the comet a second time when he was three times the age his father was when he saw the Comet. How old was Bill when he saw the Comet for the first time?
<bot>: How old was Bill when he saw the Comet for the second time? ** Bill saw the Comet for the second time when he was 30 years * 3= <<30*3=90>>90 years old.
How old was Bill when he saw the Comet for the first time? ** Comet Halley can be seen every 75 years, so Bill saw the comet for the first time when he was 90 years - 75 years = <<90-75=15>>15 years old.
#### 15