A long-range reconnaissance aircraft carries $16000\text{ kg}$ of jet fuel at takeoff. During climb to cruise altitude, it consumes $15\%$ of the takeoff fuel. During cruise, it consumes $70\%$ of the remaining fuel. How much fuel (in kg) remains in the tanks at the beginning of the descent phase?

(A) 4080 kg

(B) 2400 kg

(C) 4800 kg

(D) 3600 kg

Answer Key:

A

### 5-Stage Derivation & Analytical Solution: 1. **Stage 1: Initial State:** $W_0 = 16000\text{ kg}$. 2. **Stage 2: Post-Climb Mass:** Fuel remaining $W_1 = 16000 \times 0.85 = 13600\text{ kg}$. 3. **Stage 3: Post-Cruise Mass:** $70\%$ of remaining is burned, so $30\%$ is preserved. $W_2 = 13600 \times 0.30 = 4080\text{ kg}$. 4. **Stage 4: Selection:** Option A is the correct answer.

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