( Its 6.29 but if you are rounding to the nearest thousandths then its 6.30 x 10 to the power of -3 since jumped 3 spaces back. Remember that when it starts with a 0 , you are moving to the first digit which in this case is 6)
Since Br-80 does not exist, maybe that means Br-79 or Br-81 have very unequal abundances. For example, Br-79 may have 75% abundance whereas Br-81 may have 25% abundance.