1. **State the problem:** Factorise completely the expression $ab^2 - bc$. 2. **Identify common factors:** Look for common factors in both terms. The terms are $ab^2$ and $bc$. 3. **Find the greatest common factor (GCF):** Both terms have a factor $b$ and $c$ appears only in the second term, so the common factor is $b$. 4. **Factor out the common factor:** $$ab^2 - bc = b(\cancel{b}a\cancel{b} - c)$$ 5. **Correct the cancellation:** Actually, $b$ is common, so factor $b$ out: $$ab^2 - bc = b(ab - c)$$ 6. **Final factorised form:** $$b(ab - c)$$ This is the complete factorisation since $ab - c$ cannot be factored further.