Question

Trei mucitori ar putea executa o lucrare in 4 zile.Primul muncitor ar putea executa intreaga lucrare in 9 zile,iar al doilea muncitor - in 12 zile.In cite
zile ar putea executa aceeasi lucrare cel de-al treilea muncior lucrind singur?

Answer 1

1/9 + 1/12+ 1/x = 1/4
4/36+3/36+1/x=9/36
7/36+1/x=9/36
1/x=9/36-7/36
1/x=2/36
1/x=1/18
x=18 zile

Answer 2

1 lucrare in 4 zile - 1/4
primul - 9 zile
al doilea- 12 zile
al treilea - 1/x
1/9+1/12+1/x=1/4
[(4*1)/(4*9)]+[(3*1)/(3*12)]+1/x=1/4
4/36+3/36+1/x=1/4
(4+3)/36+1/x=1/4
7/36+1/x=1/4
Numitorul comun 36x
(7x+36)/36x=1/4
Inmultim incrucisat:
4(7x+36)=36x
28x+144=36x
-8x=-144
8x=144
x=144/8
x=18