Final answer:
To find the original number, add 18 to the result of the subtraction.
Step-by-step explanation:
To find the original number, we need to undo the subtraction that Mark did. If Mark subtracted 18 from the original number and got 40.9, we can add 18 to 40.9 to find the original number.
40.9 + 18 = 58.9
So, the original number that Mark was thinking of is 58.9.