Answer
The mass of $^{185}Re$ is equal to 185 u.
Work Step by Step
1. The sum of all abundances must be equal to 100$\$$ (1):
$$x+0.6260 = 1$$
$$x = 1 0.6260$$
$$ x = 0.3740 $$
2. Calculate the missing atomic mass:
$$( 0.3740 )( x )+( 0.6260 )( 186.956 \space u)=186.207 \space u$$
$$ ( 0.6260 )(x) = 186.207 \space u -( 0.6260 )( 186.956 \space u)$$
$$ x = \frac{186.207\space u - ( 0.6260 )( 186.956 \space u)}{0.3740 } $$
$$x = 185 \space u $$
