Step 1: A voltmeter has random errors following a normal distribution with standard deviation σ = 0.5 V. What is the probability that a single reading is within ±1.0 V of the true value? Step 2: The range ±1.0 V is ±2σ (since 2 × 0.5 = 1.0). The probability within ±nσ of the mean is erf(n/√2). Step 3: Here n = 2, so P = erf(2/√2) = erf(√2) = erf(1.414). Step 4: Using the known value (or the 95% rule), erf(1.414) ≈ 0.9545. Step 5: So about 95.5% of readings fall within ±1.0 V of the true value; only about 4.5% fall outside. Result: P ≈ 0.955. This is the origin of the two-sigma 95% confidence interval quoted in lab reports.