Jack can swim 3 laps in 7 minutes.

At this rate, how many laps can Jack swim in 28 minutes?

Jack can swim 12 laps in 28 minutes. 28 minutes is 4 times greater than the time it took him to swim 3 laps. With 28 minutes he should therefore be able to swim 4 times as many laps. 3 times 4 equals 12

He can swim 3 laps in 7 minutes.

28:7=4

4*3=12

we can conclude that in 28 minutes he can swim 12 laps.