If she can swim 2 laps in one minute, she can swim 1 lap in half the time. If you set it up as a proportion, it's easy to figure out. I'm setting up the proportions with laps on top and minutes on bottom:

. Cross multiply to get 2x = 12 and x = 6. This means that in half the time it takes to swim 2 laps, she can swim half the distance.