“That’s what Mother Gunga thinks, is it,” he said, reading. “Keep cool, young ’un. We’ve got all our work cut out for us. Let’s see. Muir wired half an hour ago: ‘Floods on the Ramgunga. Look out.’ Well, that gives us—one, two—nine and a half for the flood to reach Melipur Ghaut and seven’s sixteen and a half to Lataoli—say fifteen hours before it comes down to us.”
This is from "The Bridge Builders" by Rudyard Kipling.
I don't understand the meaning of "seven’s sixteen and a half".