Solve each equation and show your work for all them
a. n-3.4=2312.2

b. 1× =5.12
--
2

c. m + 2 = 7.8

d. 1.2 d = 6

A. n - 3.4 = 2312.2

Show Work: 2312.2
                    +    3.4
                   
             n =  2315.6

b. 1x = 5.12 

Show Work: 5.12 divided by 1 would equal to 5.12 (Note: Anything times or divided by 1 would give you the same factor that you multiplied.)
x = 5.12

c. m + 2 = 7.8

Show Work: 7.8
                   -   2
                 
            m =  7.6

d. 1.2 d = 6 

Show Work: 1.2 divided by 6 would equal to 5. 
d = 5

If you don't believe, check with an calculator, but glad to help! ^^

Whose mistake? can you give more details to what the question is asking?