If q, s, and t are all different numbers, is q < s < t?

It very well could be, but not necessarily. What if q = 10, s = 8, and t = 6?

The statement is that q is less than s, and s is less than t (which therefore also means q is less than t).

However, when we substitute (replace) all the letters with the values we assigned them, we can find that the expression becomes:

10 < 8 < 6

This clearly isn't right, as 10 is the greatest number there, and 8 is definitely less than 10 and greater than 6.