$\text{a)}$  

$\underline{280-50}+52=230+52=282$

$\underline{57+16}-20=73-20=53$

---

$\text{b)}$  

$\underline{4\cdot 25}\cdot 12=100\cdot 12=1200$

$\underline{15\cdot 4}:6=60:6=10$

---

$\text{c)}$  

$72-2\cdot \underline{5^2}=72-\underline{2\cdot 25}=72-50=22$  

$95+5\cdot \underline{3^3}=95+\underline{5\cdot 27}=95+135=230$