Lithium exists as two isotopes: Lithium-6 and Lithium-7 in the ratio of 2:25. What is the relative atomic mass of the lithium?

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Here is an example of how to calculate atomic weight.

What is different in your problem is that the relative amounts are given as a ratio, 2:25. To convert this to fractional abundance, we note that of 27 units of the Lithium,
2 will be Lithium-6 and 25 will be Lithium-7, so the respective fractions are 2/27 and 25/27. That should allow you to solve the problem.