For part A: the constant is 83.50. the variable would be c and w. and the coefficient would be 10c or 15w. 
part B: so 10(4)+15(15)+83.50 -----> 10x4= 40------> 40+15(15)+83.50---->15x15=225
40+225+83.50=348.50
part C The coefficient would change because a constant is always constant, a variable is just a letter standing in for a number which would make the coefficient the one that changes for $10 to $20.
hope that help!
        
             
        
        
        
Answer: 17/60
Step-by-step explanation:
Change all of the fractions to have a common denominator:
LCM of 10, 12, 15 is 60
You get 12/60, 25/60, and 54/60.
54/60 is what he had at the start, and so you can just subtract what he gave away
54/60-12/60=42/60
42/60-25/60=17/60
17 is a prime number so you cannot simplify the fraction
 
        
                    
             
        
        
        
Answer:
66
Step-by-step explanation:
Use the formula for the mean of a set of numbers:
                   sum of the numbers
average = --------------------------------
                      count of numbers
Here we know that the average mark was 68, so we can write:
                 
                            75 + 62 + 84 + 53 + x
average = 68 = ----------------------------------
                                            5
or                            274 + x
                   68 = -------------------
                                     5
Solving for x, we multiply both sides by 5:     340 = 274 + x.
Subtracting 274 from both sides effectively isolates x:
  340 = 274 + x 
- 274    -274
------------------------
   66      =    x
The fifth student's mark was 66.