The entries that complete the blanks are 20.39, 20.78, 21.17 and 20 + 0.39n
How to determine the total amounts?
The given parameters are
Invitation = $20
Stamps = $0.39 per stamp
So, the total amount spent is calculated as
Total = Invitation + Stamps * Number stamps
This gives
Total = 20 + 0.39 * Number stamps
For stamps 1, 2, 3 and n, we have
Total = 20 + 0.39 * 1 = 20.39
Total = 20 + 0.39 * 2 = 20.78
Total = 20 + 0.39 * 3 = 21.17
Total = 20 + 0.39 n
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