Question

7 grainzi de brad si 12 grinzi de stejar cantaresc  impreuna  750 kg. 3 grinzi de brad si 8 grinzi de stejar cantaresc impreuna 450 kg . Cate kg cantareste o grainda de brad si cate kg cantareste o grinda de stejar?

Answer 1

x-grinzile de brad
y-grinzile de stejar

7x+12y=750 |-3
3x+8y=450   |7

-21x-36y=-2250
21x+56y=3150
/ 20y= 900
y=45 kg

7x+12*45=750
7x+540=750
7x=750-540
7x=210
x=30kg

Answer 2

7 grinzi brad.................12 grinzi stejar     = 750 kg
3 g. brad.......................8 g. stejar           = 450 kg  facem scaderea
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4 g. brad.......................4 g. stejar           = 300kg       : 4 (impartim la 4)
1 g. brad + 1 g. stejar = 75 kg
1 b = 75 - 1 s
3 b + 8 s = 450  inlocuim pe b
3(75 - 1 s) + 8 s = 450
225 - 3 s + 8 s = 450
5 s = 225
s = 225 : 5 
s = 45 kg o grinda stejar
b = 75 - 45
b = 30 kg o grinda brad