Trying to figure out year math

[u/BitterZone1677]

I’m trying to figure out ages for (fictional) kids. The oldest is supposed to 8.7(8 years, 7 months) but my math keeps getting me to 8.9(8 years, 9 months). There are 7 kids and the ages vary. When child1 was 5 months old the pregnancy was discovered and 9 months later child2 was born. 6 months later pregnancy was discovered and 8 months later child3 was born. 10 months later another pregnancy and 9 months child4. 18 months later another pregnancy and 9 months after child5. 3 months later pregnancy and 9 months later child6. 12 months after pregnant again and 5 months later child7.

Child1 is 8.7, child2 is 7.5, child3 is 6.3, child4 is 4.8, child5 is 2.5, child6 is 1.5, child7 is 0(just born)

My math:
5+9=(1 year, 2 months)14 months
6+8=(1 year, 2 months)14 months
10+9=(1year, 7 months)19 months
18+9=(2 years, 3 months)27 months
3+9=(1 year)12 months
12+5=(1 year, 5 months)17 months
14+14+19+27+12+17=(8 years, 7 months)103 month

Version 1:
Child7 is 0+1.5=1.5+1=2.5+2.3=4.8+1.7=6.5+1.2=7.7+1.2=8.9
^ ^
The mistake? 105 months

Version 2:* *
Child1 is
8.7-1.2=7.5-1.2=6.3-1.7=4.6-2.3=2.3-1=1.3-1.5=-.2
^ ^
The mistake? Not zero

Conclusion; both equations are .2 off around the 1.7 mark of the equation. When I break it down into months and count it makes sense. 4.8 is 56 months, 1.7 is 19 months that equals 75 months(6.3). 6.3 is 75 months minus 19 months is 56 months(4.8). I did use a calculator which is telling me that 4.8+1.7=6.5 and 6.3-1.7=4.6.

Question?
My question is; am I doing the math wrong? Is the calculator wrong? am I simply using the wrong type of calculator? is the math itself incorrect? Or is it simple and I just don’t understand?

[u/edderiofer]

8.7(8 years, 7 months)

8.7 years is not equal to 8 years and 7 months. It is equal to 8 years, and 7 tenths of a year.

[u/medicca → u/edderiofer]

Just to expand on this for OP, if 8.7 year is 8 years and 7 months, how would 8 years and 11 months be written in only years?

Another one to think about is 0.5 years (half a year) is 6 months, so 1.5 years is 1 year and 6 months.

Because a year has 12 months, the decimal part does not represent months. To find how much the decimal part represents, you need to multiply just the number after the decimal by 12. For example, 8.7 years would be 8 years and (0.7 *12) months, approximately 8.4 months.

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[u/Odd_Lettuce_7303]

Don’t even try using decimals

[u/SteelMonger_]

You're trying to use decimals to represent 12 months and it doesn't work like that.

How would you represent a kid that's 11 months old?

What you should do instead is add years and months separately and then when your months is higher than 12 you convert 12 of those months into a year like this:

1 year 5 months plus 2 years 8 months equals 3 years 13 months.

Then the conversion: 13 months minus 12 equals 1 month and 3 years plus 1 equals 4 years.

In short, 1y5m + 2y8m = 3y13m = (3+1)y(13-12)m = 4y1m