Natural lithium is mostly lithium-7. For some special uses, lithium is enriched, so that it is 30 %…

Step 1. Weight each isotope. $6 \times 30 = 180$ and $7 \times 70 = 490$.

Step 2. Add and divide. $\dfrac{180 + 490}{100} = 6.7\ \text{u}$.

Step 3. Compare. Natural lithium has much less lithium-6, so its average is closer to $7\ \text{u}$. The enriched lithium is lighter.