10. a×b=240
a×(b+5)=300
a×b+5a=300
240+5a=300
5a=300-240=60 => a=60÷5 => a=12
b=240÷12 => b=20
11. 7x+14x+...+126x=7×9×19
Dam factor comun pe x:
x(7+14+...+126)=1197
S=7+14+...+126
(126-7)÷7(ratia)+1=18 termeni
S=[18(7+126)]÷2
S=(18×133)÷2
S=9×133=1197
x×1197=1997 => x=1
Sper ca te-am ajutat!! :)))
