Take a look at the options for the Brush tool and you'll see two settings: Opacity and Flow. To change the opacity from the keyboard just press the first number of the percentage you want (e.g. 5 for 50%, 6 for 60%, etc). But how do you change the percentage setting for Flow? Just add in the Shift key: use Shift and the first number of the percentage you want.
Bonus tip: if the Airbrush option is turned on, the shortcuts flip flop: use only the number to change Flow, use Shift and the number to change Opacity.
Bonus tip 2: To turn the Airbrush option on and off from the keybard: Option Shift P (PC: Alt Shift P)