When a decimal number has trailing zeros ahead of the decimal point, the precision is always in doubt. Some try to ease the ambiguity a bit by declaring that the presence of a decimal point (as in 40.) means the zeros are all significant.
If we use that rule, then the precision of 40 ft is ±5 ft, because no decimal point is present and the least-significant non-zero digit is in the tens place.
(It is not unreasonable to assume that a measurement of 40 ft can be made to the nearest foot, so the precision might be ±0.5 ft. It depends on the technology involved.)