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c = 3/5 or 0.6

Step-by-step explanation:

5c + 16.5 = 13.5 + 10c

-5c                      - 5c

16.5 = 13.5 + 5c

-13.5  - 13.5

3 = 5c

3 ÷ 5 = c

c = 3/5

  (a) 25

  (b) (5+4)*2 +6 -2*2 -1

  (c) see below

Step-by-step explanation:

(a) evaluating exponents:

  6*7 -9*9 +64

Evaluating products:

  42 -81 +64

Evaluating sums:

  -39 +64

  25

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(b) The last three terms of the expression sum to +1, and it is fairly easy to modify the first two terms to make one term that has a value of 18. Then the expression evaluates as 18+1 = 19.

  (5+4)*2 +6 -2*2 -1

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(c) Evaluating parentheses:

  = 9*2 +6 -2*2 -1

Evaluating products:

  = 18 +6 -4 -1

Evaluating sums (left to right):

  = 24 -4 -1

  = 20 -1

  = 19