If the average marks of three batches of 30, 50 and 70 students is 55, 70 and 70 respectively, then the average marks of all the students is :

67

Sum of marks of 30 batch students = 30 × 55 = 1650

Sum of marks of 50 batch students =50 × 70 = 3500

Sum of marks of 70 batch students = 70 × 70 = 4900

Average of all = \(\frac{1650 + 3500 + 4900 }{150}\) 

= 67