3st + 5c = 65 =>3st = 65 - 5c =>
=> st = (65 - 5c) : 3
2c + 3p = 26 => 3p = 26 - 2c =>
=> p = (26 - 2c) : 3
Inlocuim in relatia :
7st + 2p = 78 , in functie de c
7 × (65 -5c) : 3 + 2 ×(26 - 2c) : 3 = 78 | × 3
7 × (65 - 5c) + 2 × ( 26 - 2c) = 78 × 3
455 - 35c + 52 - 4c = 234
35c + 4c = 455 + 52 - 234
39c = 273
c = 273 : 39
c = 7 lei costa un caiet
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p = (26 - 2c) : 3
p = (26 - 2 × 7) : 3
p = (26 - 14) : 3
p = 12 : 3
p = 4 lei costa un pix
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st = ( 65 - 5c) : 3
st = (65 - 5 × 7) : 3
st = (65 - 35) : 3
st = 30 : 3
st = 10 lei costa un stilou
_____________________
verificare:
3 × 10 + 5 × 7 = 30 + 35 = 65 lei
2 × 7 + 3 × 4 = 14 + 12 = 26 lei
7 × 10 + 2 × 4 = 70 + 8 = 78 lei
