Billie was very quick on the uptake, and knew immediately what the man was after.
Advanced English Dictionary
+noun [U, sing.] ~ (of sth) (technical) the process by which sth is taken into a body or system; the rate at which this happens: the uptake of oxygen by muscles Idioms: be quick / slow on the uptake to be quick/slow to understand sth: (informal) Is he always this slow on the uptake?
Random quote: In the middle of every difficulty lies opportunity.: Albert Einstein