Calculate the average atomic mas of an unknown element that has two naturally-occurring isotopes.

the first isotope occurs 75.47% of the time and has a mass of 248.7 a. m.u.
the second isotope occurs 24.53% of the time and has a mass of 249.4 a. m.u.

248.9 uma

Explanation:

We calculate the average atomic mass using the following formula

This formula depends on how many isotopes the atom has.

If the atom had three isotopes another term would be added to the formula

8 m/s because for him to get the same amount of energy as ray is putting out, bobby, who weighs half as much as ray, needs to run twice as fast.