Not even sure how to start with this one or how to calculate

Remember when you learning about the "Least Common
Multiple" and you wondered when in the world you would
ever need to use it ?
Well, here you are !   Hello !

The next time they will cross the starting line together is the
LCM (Least Common Multiple) of 30 and 40 .

Do you remember how to find it ?
Here's one easy way: 

Count by 30s until you come to a number that's divisible by 40 .

Laura's first lap. . . . . . . . 30 seconds . . . not divisible by 40
Laura's two laps . . . . . . . 60 seconds . . . not divisible by 40
Laura's three laps. . . . . . 90 seconds . . . not divisible by 40
Laura's four laps. . . . . . 120 seconds . . . divisible by 40

After 120 seconds, Laura has completed four laps,
Shelby has completed only three laps.

What exactly is your question?