Answer:
21.
Explanation:
For questions like this, it's easiest to work your way backwards from the answer to get the original number.
In this case, you subtract 12 from 54, which gives you 42 (54-12=41)
Then, you divide by 2, the opposite of "doubling". 42 ÷ 2 = 21.
Since there are no other operations, your original number is 21.
To confirm, just go through the problem again starting with 21 and seeing if you get to 54.
21, doubled (42), add 12 = 54.