Question

Determinaţi numărul numerelor de forma ab1 ( cu liniuţă sus) cu a+b<5

Answer 1

a+b ∈{1,2,3,4,}⇒a ∈{1,2,3,4} 
b∈{3,2,1,0}
ab1∈{131,121,111,101,211,201,221,311,301,401}