The average weight of 20 students of a class is 60 kg. A student whose weight was 45 kg left the class and new student replaced the old one. Hence, the average weight of students increased by 1 kg. Find the ratio of old and new student's weight.?

Option 1 : 9 : 13 

Given 

Average weight of 20 students of a class = 60 kg

Weight of old student who left = 45 kg

Increase in average weight of students = 1 kg

Formula Used

Average = Sum of observation/No. of observation

Calculation

Total weight of 20 students = 20 × 60 kg = 1200 kg

New average of weight of the students will be = 60 + 1 = 61

Let the weight of new student = x kg

So According to question

⇒ New average weight of students = 61

⇒ (1200 – 45 + x)/20 = 61

⇒ 1200 – 45 + x = 61 × 20 

⇒ x = 1220 - 1200 + 45

⇒ x = 65 kg

Ratio of old and new student's weight = 45 : 65 = 9 : 13

∴ The Required ratio is 9 : 13