Let x be the number Henry is thinking of.
The number is doubled and got an answer of 4.
That means:
2 · x = 4
or
2x = 4
This is the equation when Henry doubled the number and got 4 as an answer.
Solve it by dividing both sides by 2.
2x ÷ 2 = 4 ÷ 2
x = 2
The original number Henry was thinking was 2.
Cheers!