1. **Problem:** Simplify the expression for the cost of DVDs after a 25% markup given by $c + 0.25c$. 2. **Formula:** When adding like terms, add their coefficients: $a x + b x = (a+b)x$. 3. **Work:** $$c + 0.25c = (1 + 0.25)c = 1.25c$$ 4. **Explanation:** We combined the terms by adding the coefficients 1 and 0.25 to get 1.25, so the simplified expression is $1.25c$. **Final answer:** $1.25c$