A Puzzle A Day: 2026-04-12

Correct: 16 minutes 40 seconds

Let 'W' be the total work required to test one software build. 1. **Calculate the rate of one old rig:** The 5 old rigs complete 'W' work in 20 minutes. So, the combined rate of 5 old rigs is W/20 (work per minute). The rate of a single old rig (R_old) is (W/20) / 5 = W/100 (work per minute per rig). 2. **Calculate the rate of one new rig:** Each new rig is 50% more efficient, meaning it processes tests 50% faster. So, its speed is 1.5 times the old rig's speed. Rate of a single new rig (R_new) = R_old * 1.5 = (W/100) * 1.5 = 1.5W/100 = 3W/200 (work per minute per rig). 3. **Calculate the combined rate of 4 new rigs:** The 4 new rigs work simultaneously, so their combined rate (R_total_new) is 4 * R_new. R_total_new = 4 * (3W/200) = 12W/200 = 3W/50 (work per minute). 4. **Calculate the time to complete the work:** Time = Total Work / Combined Rate = W / (3W/50) = 50/3 minutes. 5. **Convert to minutes and seconds:** 50/3 minutes = 16 and 2/3 minutes = 16 minutes and (2/3 * 60) seconds = 16 minutes and 40 seconds. **Common pitfalls leading to other choices:** * **10 minutes 40 seconds (Choice A):** This results from an incorrect initial assumption that time is directly proportional to the number of rigs (20 * 4/5 = 16 minutes), and then dividing by 1.5 for efficiency (16 / 1.5 = 10.66... minutes). * **12 minutes 30 seconds (Choice B):** This occurs if '50% more efficient' is misinterpreted as taking '50% less time' for the equivalent work. If 5 old rigs take 100 'rig-minutes', a new rig would take 50 'rig-minutes'. Then, 50 'rig-minutes' / 4 rigs = 12.5 minutes. * **25 minutes (Choice D):** This is calculated by only adjusting for the change in the number of rigs, completely ignoring the efficiency gain (20 minutes * (5 rigs / 4 rigs) = 25 minutes).