Salut !
[tex] \frac{x}{3} = \frac{y}{4} = \frac{z}{5} = > x = 3k;y = 4k;z = 5k[/tex]
[tex]x + y + z = 1440[/tex]
[tex]3k + 4k + 5k = 1440[/tex]
[tex]12k = 1440[/tex]
[tex]k = 1440:12[/tex]
[tex]k = 120[/tex]
[tex]x = 120 \times 3 = 360[/tex]
[tex]y = 120 \times 4 = 480[/tex]
[tex]z = 120 \times 5 = 600[/tex]
[tex]600 + 480 + 360 = 1440[/tex]
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