Answer:
36 kg
Step-by-step explanation:
Let x = amount of green tea she needs to use.
Black tea: 18 kg, $3.5/kg
Green tea: x kg, $2.6/kg
Mixture: (x + 18) kg; $2.9/kg
3.5 × 18 + 2.6 × x = 2.9 × (x + 18)
2.6x + 63 = 2.9x + 52.2
-0.3x = -10.8
x = 36
Answer: 36 kg