There is no mistake .. Marks distribution is correct.
There were three questions in Q no. 7 i.e 7(a) , 7(b) and 7(c)
7(a) was a 8 Marks question, 7(b) 4 marks and 7(c) 4 marks = TOTAL 16 marks question.
In 7(a) there were 3 question of 4 marks each , out which you had to do only 2 question . So its simple.