13). z = ( 9/13)(C)
Solve for C:
Step 1: flip Equation:
9/13 c = z
Step 2: Divide both sides by 9/13
9/13 c/9/13 = z/9/13
C = 13z/9
 Answer: Letter Choice, (D), C = 13z/9
12.). Let " h" be the number of hikes:
 Expense of First Club:
C * h = 20 + 5h
Expense of Second CluB:
C * h = 7h
20 + 5h < 7h
20 < 2h
h > 10 hikes
Answer: The First Club is Less Expensive, If it is more than Ten Hikes.
14.).  Co-Efficient of Friction Formula: 
 ma = F - mw 
 + mw + mw (cancel out +mw)
 Mw + ma = F - ma 
 -ma -ma (Cancel out -ma)
 Divide: Both sides by w:
 Mw/w = F - ma/w
Therefore,  Answer: Letter Choice, (D), M = F - ma/w
Hope that helps!!!! Have a wonderful day!!!! : )