a+a+1+a+2+a+3+a+4 = 1 625
5×a + 10 = 1 625
5×a = 1 625-10
5×a = 1 615
a = 1 615:5
a = 323
323; 324; 325; 326; 327 .
325 nr mijlociu
325 × 3 = 975
10 : 9 = 1 rest 1
20 :9 = 2 rest 2
.
.
.
80:9 = 8 rest 8
10+20+30+40+50+60+70+80 =
= 90×4 =
= 360 (suma)