Let the number they are thinking of be X.
They double x , which would be 2x.
They then add 20, which would be 2x +20
So you have 2x + 20 = 27.7
Now we can solve for x:
Subtract 20 from both sides:
2x = 7.7
Divide both sides by 2:
x = 7.7 / 2
x = 3.85
The original number was 3.85