If the difference in longitude of two places is 4°45′, what will be the difference in time?

Correct answer: b 19 minutes

4° × 4 = 16 minutes and 45′ × 4″ = 180 seconds = 3 minutes; total 19 minutes.

Why the other options are wrong

• a 18 minutes: 4 × 4 = 16 minutes and 45′ × 4 s = 3 minutes; the sum is 19, not 18.
• c 20 minutes: This rounds 4°45′ to 5° (5 × 4 = 20); the exact working gives 19 minutes.
• d 21 minutes: This multiplies the minutes part by 4 minutes; the minutes part is multiplied by 4 seconds.