decimals must be estimates unless they have multiples of 2 or 5 in the mantissa
1.857142857 + 2.142857142 =3.9999999999 estimate 4.00000000
1 6/7 + 2 1/7 = 4 exact
1.1250000000 + 2.875 = 4.000 +- only 3 significant digits so best estimate possible
1 1/8 + 2 7/8 = 4 exact
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