There was a net immigration to the country of 50,000-30,000=<<50000-30000=20000>>20,000
So the total number of people before birth is 300,000+20,000=<<300000+20000=320000>>320,000
That means 320,000/8=<<320000/8=40000>>40,000 get pregnant
So 40,000/4=<<40000/4=10000>>10,000 have twins
So that means they have 10,000*2=<<10000*2=20000>>20,000 children
There are 40,000-10,000=<<40000-10000=30000>>30,000 children from single births
So the total number of people is 320,000+30,000+20,000=<<320000+30000+20000=370000>>370,000
#### 370000