# Small doubt in asynchronous counter

I was trying to understand the ripple effect in asynchronous counter and I came across this figure:-

In all the problems solved in my book, they consider the minimum clock period required for proper functioning of counter as the sum of the delays caused due to these flip flops.

If we consider each delay as 5ns in the figure then the answer would be 15ns (minimum clock period time) according to my book.

My doubt is if it is exactly 15ns, then after 15ns, Q0 will toggle and we would get "1". So at the end we can't get 1000 as output as by that time Q0 would have toggled and we would get 1001 skipping 1000.

So I don't understand how clock period of 15ns would solve the problem of rippling.