Correct Answer - Option 1 : 66

**Given **

Three batches of student = 50, 60, 70

The average marks of three batches = 65, 50, 80,

**Formula Used **

Average = sum of data/No. of data

**Calculation **

Sum of marks of 50 student = 50 × 65 = 3250

Sum of marks of 60 student = 60 × 50 = 3000

Sum of marks of 70 student = 70 × 80 = 5600

Average marks of all students = Sum of markets obtained by all students/Total number of students

⇒ (3250 + 3000 + 5600)/(50 + 60 + 70)

⇒ 11850/180 = 65.83 = 66(approx.)

**∴ The average marks of all students is 66.**