A fictitious element X is composed of 10.0 percent of the isotope 55 X, 20.0 percent of the isotope 56 X, and 70.0 percent of the isotope 57 X. Estimate the atomic mass of element X.

56.6 g/mol.

Explanation:

Assuming here that we have 100 atoms of each element. The percentage tells us that we have 10 atoms of the 55 isotope, 20 of the 56, and 70 of the 57.  

So the total is 10*55 + 20*56 + 70*57 = 5,660. And to get the average, we just divide by the total number of atoms (100).  

Atomic mass = 5660/100 = 56.6  

i think its 100.

hope this !