Why did england become vulnerable to attack around the beginning of the fifth century

Because the previous military and political authority of the country: the Roman Empire, had left the island, and this created a power vacuum that could be capitalized by foreign invaders.

The foreign invaders that capitalized this power vacuum were the Angles, the Saxons, and the Jutes, all of them germanic peoples coming from modern-day Denmark. These groups invaded England and defeated the local Briton population which was confined to Wales, and Cornwall.

post the grid otherwise i cant .

also it sounds like a math question not history.

well the fact it is basically a peninsula allowed trade to come from many places, thus allowing cultural diffusion to take place. india has seasonal monsoons that merchants travel back and forth from india using the monsoons wind patterns