Question

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Answer 1

1ª caja = n  manzanas
2ª caja = n+7  manzanas
3ª caja = n-5  manzanas

Ecuación:
n + n+7 + n-5 = 77

3n = 77 -7 +5

n = 75/3 = 25 manzanas en la caja 1

Saludos.