275 + 360 = 360 + (a)  

... + 285 = 285 + 455

(a)  

180 + ... = 700 + 180

(a)  

220 + 630 = ... + 220

(a)  

500 + 270 = 270 + ...

(a)  

... + 170 = 170 + 360

(a)  

380 + ... = 370 + 380

(a)  

240 + 270 = ... + 240

(a)  

410 + 250 = 250 + (a)