a)
1 +50=51
2+49=51
3+48=51
4+47=51
5+46=51
6+45=51
7+44=51
8+43=51
9+42=51
10+41=51
11+40=51
12+39=51
13+38=51
14+37=51
15+36=51
16+35=51
17+34=51
18+33=51
19+32=51
20+31=51
21+30=51
22+29=51
23+28=51
24+27=51
25+26=51
26+25=51
(51-1):2=50:2=25
25+1=26 de perechi sunt in acest sir succces