Afictitious element x is composed of 10.0 percent of the isotope 55x, 20.0 percent of the isotope 56x, and 70.0 percent of the isotope 57x. calculate the weighted atomic mass of element x to the nearest tenth.

56.6

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  

most likely he is cheating on you

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.