The ratio of the number of tables and chairs in a tuition centre was 3 : 7. After another 20 tables and 280 chairs were added, the ratio became 2 : 7. How many tables and chairs were there in the centre at first?

Tables Chairs

At first 3 units 7 units

+20 +280

In the end 2 parts 7 parts

Make the parts the same -----> 2 parts x 7, and 7 parts x 2, ie.

Tables Chairs

At first 3 units x7 7 units x 2

+20 (x 7) +280 (x 2)

In the end 2 parts x 7 7 parts x 2

21 units + 140 = 14 parts

14 units + 560 = 14 parts

Since both are 14 parts -----> 21 units + 140 = 14 units + 560

7 units = 420

1 unit = 60

Tables at first = 3u = 3 x 60 = 180

Chairs at first = 7u = 7 x 60 = 420

Answer: At first, there were 180 tables and 420 chairs in the centre.