In this type of calculations, we decompose 13 by checking the lowest powers of the base, that is 40. for example we check 40^2, or 40^3 and compare it to 85
Notice
40*40*40=64,000
so we check how many time does 85 fit into 64,000:
64,000/85=752.94
85*753=64,005;       64000-64,005=-5
this means that 

thus

Answer: 10 (mod85)
Remark, the set of all solutions is:
{......-75, 10, 95, .....}, that is 85k +10
 
        
        
        
Answer:
(a)There are 24 cakes in total
(b)There are 15 pieces of cakes left
Step-by-step explanation:
Let the total cakes=x
Let number of cakes eaten by Mindy=m
Let number of cakes eaten by Troy=t
Now:
m=3
Since m+t=9
3+t=9
t=9-3=6
The Number of Pieces of Cake Left in Total=x-(t+m)
Since Troy had  of the total cake
 of the total cake


x=6X4=24
(a)There are 24 cakes in total
(b)The Number of Pieces of Cake Left in Total=x-(t+m)=24-9=15 cakes