Question

Aduna treimile tuturor numerelor naturale mai mici ca 16

Answer 1

3:3=1
6:3=2
9:3=3
12:3=4
15:3=5
1+2+3+4+5=15

Answer 2

$\frac{1}{3} + \frac{2}{3} + \frac{3}{3} +...+ \frac{15}{3} = \\ \frac{1}{3} (1+2+3+...+15)= \\ \frac{1}{3} (15*16)/2= \\ \frac{1}{3} *15*8=40$