Calcium has three different isotopes. One has a mass of 35.00 amu; another has a mass of 41.00 amu; and another has a mass of 40.00 amu. Which isotope is the most abundant of the three?

The quoted atomic mass on the Periodic Table is the weighted average of the individual isotopes. The most abundant isotope should be ""^40Ca.
It follows that whatever atomic mass is quoted on the Periodic Table, the most abundant isoptope of that element must make the greatest statistical contribution. Calcium has an atomic mass of $40.07$ $g \cdot m {l}^{-} 1$. Without looking up the isotopic distribution, clearly ""^40Ca is the most abundant isotope.