Magnesium has 3 naturally occurring isotopes: 24 mg (23.985 amu) with 78.99% abundance,
25 mg (24.986 amu) with 10.00% abundance, and a third with 11.01% abundance.
look up the atomic weight of magnesium, and then calculate the mass of the third isotope.

The mass of third isotope is 25.98 amu.

Explanation:

Mass of Mg²⁴ = 23.985 amu

Mass of Mg²⁵ = 24.986 amu

Abundance of Mg²⁴ = 78.99 %

Abundance of Mg²⁵ = 10%

Abundance of third isotope = 11.01%

Atomic weight of magnesium = 24.305

Solution:

Atomic mass = ( % age of first isotope × its atomic mass) + (% age abundance of second isotope × its atomic mass) + ( % age of third isotope × its atomic mass ) / 100

Now we will put the values in formula.

24.305  = (78.99 × 23.985 ) + (10 ×24.986 ) + ( 11.01 × X) / 100

24.305 = 1894.56 + 249.9 +( 11.01 × X ) / 100

24.305 × 100 = 2144.46 +  ( 11.01 × X)

2430.5  -  2144.46  =  (11.01 × X)

286.04  / 11.01 = X

25.98  = X

The mass of third isotope is 25.98 amu.