after being joined by its tributaries,how does ganga flow into the bay of bengal?(5)

Following points can help you better:
Ganga originates from a Glacier named Gangotri in Uttranchal.
Ganga flows across the plains of Northern India, these plains are also known as Gangetic Plains and through these plains it finally reaches the BAY Of Bengal.
Before draining into the BAY OF Baengal it's route is as under:

• Gganga follows an 800 km curving course passing through the city of Kanpur before it is being joined from the southwest by the Yamuna at Allahabad.
• On it's way it is joined by some numerous rivers such as the Kosi, Son, Gandaki and Ghaghra, the Ganges forms a formidable current in the stretch between Allahabad and Malda in West Bengal.
• As it's journey continues it passes the towns of Mirzapur, Buxar , Varanasi, Patna and Bhagalpur. At Bhagalpur, the river meanders past the Rajmahal Hills, and begins to run south.
• At Pakur, the river begins its attrition with the branching away of its first distributary, the Bhāgirathi – Hooghly, which goes on to form the Hooghly River. Near the border with Bangladesh the Farakka Barrage, built in 1974, controls the flow of the Ganges, diverting some of the water into a feeder canal linking the Hooghly to keep it relatively silt-free.
• Fanning out into the 350 km wide Ganges Delta, it finally empties into the Bay of Bengal.