Acertain protein was found to contain 0.870% cobalt by mass. determine the minimum molecular mass of the protein in amu.

0.870% Cobalt by mass can be written as a ratio:
0.870 amu Co : 100.000 amu (total protein)
Therefore,
55.85 amu Co = (100.000 amu total protein) / (0.870 amu Co) = 698881.25 amu 

rounded  to 3 significant figures :
We get, 
69,900 amu