LCM of 2 and 7 is 14

Long rows: 1/2 = 7/14
Short rows: 3/7 = 6/14

6u = 18 short rows
1u = 3 short rows
7u = 21 short rows

21 short rows = 7 long rows

Since each long row has 18 more chairs than each short row,
we take 18 chairs from each of the 7 short rows to form 14 more rows (21 - 7)
The idea is to form 21 short rows just like the 21 short rows already present

So 18 x 7 = 126

21 - 7 = 14
Number of chairs in each short row
= 126/14
= 9
Number of chairs in each long row
= 18 + 9
= 27 (Ans)