Ratios

They meet every 15 seconds, so in 15 seconds both of them travel the whole length of the circle. 

In 15 seconds Jess travels 15/40 of the circle. So 3/8. That means Jane must travel 5/8 of the circle in the same time.

Let x be the time Jane taxes to complete a lap. We know in 15 seconds she completes 5/8 so x * (5/8) = 15  ->  x= 24.