Bluebulb Projects' The Measure of Things presents:
31 is about two-fifths the number of Submarines in the US Navy.
In other words, it's 0.430 times the count of Submarines in the US Navy, and the count of Submarines in the US Navy is 2.30 times that amount.
(a.k.a. Subs) (includes all types, all classes) (2015 figures)