4.425 cm
1) Gathering the data from the question
1st month = 1.775 cm
2nd month =1.775 +1.45
3rd month = 1.775 +1.45+1.2
2) So to find that out, we need to sum it all, dot over the dot. For the nonexisting digits, we can add zero (in bold)
3rd month = 1.775 +1.45+1.2
1.775
1.450
1.200
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4.425 cm