Linecard port density subscription ratio

I have a C4507 switch at my company with line card WS-X4712-SFP+E. Reading through Cisco datasheet I came across below points which I have hard time understanding or can say calculating -

• 48gigabits per-slot capacity

• Bandwidth is allocated across four 3-port groups, providing 12Gbps per port group (2.5:1)

I am scratching my head with the last point and how to decode the 2.5:1 ratio if I need to use both 10G and 1G in this linecard. Can someone help me in layman terms please?

Thanks in advance.