The two numbers a question gives you

1. A question supplies the mass of each isotope and how common that isotope is.
2. An isotope's relative isotopic mass is the same number as its mass number.
3. Abundance may be given as a percentage, as a decimal fraction, or as a count of atoms detected.
4. The more abundant isotope pulls the average towards its own mass.
5. Relative atomic mass has no unit, because it compares one mass with another.

The method: multiply, add, then divide by the total abundance

1. Write each isotopic mass and its abundance side by side before calculating anything.
2. Multiply each isotopic mass by its own abundance.
3. Add all of those products together.
4. Divide by the total abundance: $$A_r=\frac{(m_1\times a_1)+(m_2\times a_2)}{a_1+a_2}$$
5. The denominator is $100$ when the abundances are percentages that add to $100$.
6. Extend the same pattern for three or more isotopes by adding one product per isotope.

Worked example: weighting three isotopes

1. An element has isotopes of mass $24$, $25$ and $26$, with abundances of $79\%$, $10\%$ and $11\%$.
2. The abundances add to $100$, so the denominator is $100$.
3. Work out the three products separately before adding, so that a slip in one does not hide inside the total.

Checking your answer

1. Your answer must lie between the lowest and the highest isotopic mass in the data.
2. It must sit nearer the mass of the most abundant isotope.
3. Do not round the intermediate products, because rounding early shifts the final value.
4. Do not take a plain average of the masses, because the isotopes are not equally abundant.
5. Give $A_r$ with no unit, and to the precision the question asks for.