Question

I am a number
Tell my identity
Take me seven times over
And sad a fifty
To reach a triple century
You still need forty

Answer 1

let the no. be x

7x+50+40=300
7x=300-90
x=30

Answer 2

Let the number be n.

According to the question,  

7n + 50 + 40 = 300

⇒7n + 90 = 300

⇒7n = 300 − 90

⇒7n = 210

⇒ n = 7/210

⇒ n = 30

Thus, the required number is 30.