I am a number, Tell my identity Take me seven time over and add a fifty

Question: I am a number,
Tell my identity
Take me seven time over
and add a fifty
to reach a triple century
you still need 40​

Solve the riddle :

Answer:  Let the number be x

7x + 50  + 40 = 300

7x + 90 = 300

7x = 210

x = 30