All roentgenium isotopes are extremely unstable and radioactive; in general, the heavier isotopes are more stable than the lighter. The most stable known roentgenium isotope, 282Rg, is also the heaviest known roentgenium isotope; it has a half-life of 2.1 minutes. (The unconfirmed 286Rg is even heavier and appears to have an even longer half-life of about 10.7 minutes, which would make it one of the longest-lived superheavy nuclides known; likewise, the unconfirmed 283Rg appears to have a long half-life of about 5.1 minutes.) The isotopes 280Rg and 281Rg have also been reported to have half-lives over a second. The remaining isotopes have half-lives in the millisecond range. The undiscovered isotope 287Rg has been predicted to be the most stable towards beta decay; however, no known roentgenium isotope has been observed to undergo beta decay. The unknown isotope 277Rg is also expected to have a long half-life of 1 second. Before their discovery, the isotopes 278Rg, 281Rg, and 282Rg were predicted to have long half-lives of 1 second, 1 minute, and 4 minutes respectively; however, they were discovered to have shorter half-lives of 4.2 milliseconds, 17 seconds, and 2.1 minutes respectively. Similarly, the measured half-life of the unconfirmed 283Rg of 5.1 minutes, while long, is less than the 10 minutes that had previously been predicted for it.
How many minutes of a half-life does the unconfirmed 286Rg and 283Rg appear to have combined?
15.8