Problem with acer keyboard .?

Whenever i have num lock enabled ,it allows me to type numbers using the external numeric pad . But at the same time , when I press keys like m, u on the Acer Aspire 9300 Keyboard , numbers appear .

The question is ,how do I type on my numeric pad and at the same time , type letter on my keyboard ? Im aware that this is not a brand problem with the laptop or external numeric Pad .