Answer: her breaststroke speed is 30 laps per hour.
Step-by-step explanation:
Let b represent Justine’s butterfly speed.
Let r represent Justine’s breaststroke speed.
Justine swims freestyle at a pace of 150 laps per hour. If Justine’s freestyle speed is three times faster than her butterfly speed, it means that
3b = 150
b = 150/3
b = 50 laps per hour
Justine swims breaststroke 20 laps per hour slower than she swims butterfly. This means that
r = b - 20
r = 50 - 20
r = 30 laps per hour