13a) 1a divizibil cu 12 => a=2
21 y 2*21 7
--- = ---- => y= ---------- = ----
12 2 12 2
13b) b cel mai mic nr prim b =2
2 14 2*42
--- = ------ => y= -------- = 6
y 42 14
13c) c5 divizivil cu 15 => 15; 45; 75 c=1, c=4, c=7
c=1 => y = 15 * 4 / 51
c=4 => y = 45 * 4 / 54
c=7 => y = 75 * 4 / 57
