Answer: To find the value of the number that was removed, we can set up equations based on the given information.
Let's assume the four numbers are A, B, C, and D.
The mean of the four numbers is 18:
(A + B + C + D) / 4 = 18
If one of the numbers is removed, the mean of the three remaining numbers is 17:
(A + B + C) / 3 = 17
Now, let's solve these equations:
From equation 1, we have:
A + B + C + D = 72
From equation 2, we have:
A + B + C = 51
By subtracting equation 2 from equation 1, we can find the value of D:
(A + B + C + D) - (A + B + C) = 72 - 51
Simplifying, we get:
D = 21
Therefore, the value of the number that was removed is 21.