When subtracting 9.3 from a certain number, the result is 68.91, as seen below. What number should go in the box to complete the subtraction problem

7.21

- 9.3

68.91

To answer this item, we let x be the number that is not unknown in this item such that the situation in this problem can be expressed as follows,

x - 9.3 = 68.91

Adding 9.3 to both sides of the equation,

x - 9.3 + 9.3 = 68.91 + 9.3

Simplifying,

x = 78.21

The answer to this item is 78.21, this I think is the first choice except that it is mistakenly written as 7.21 only.