8 persons can build a wall 140 m long in 42 days. In how many days can 30 persons complete a similar wall 100 m long?

As per given question,
8 persons can build a wall 140 meter long in 42 days.
1 persons can build a wall 140 meter long in 42 x 8 days.
1 persons can build a wall 1 meter long in 42 x 8 / 140 days.
30 persons can build a wall 1 meter long in 42 x 8 / 30 x 140 days.
30 persons can build a wall 100 meter long in 42 x 8 x 100 / 30 x 140 days.
30 persons can build a wall 100 meter long in 42 x 8 / 3 x 14 days.
30 persons can build a wall 100 meter long in 42 x 8 / 42 days.
30 persons can build a wall 100 meter long in 8 days.