A cold-storage unit for Nashik grapes reads 4°C, and its compressor lowers the reading by 2°C every hour. After how many hours does it show −10°C?

The reading must change by (−10) − 4 = −14°C in all. At −2°C per hour that takes (−14) ÷ (−2) = 7 hours, and the same signs make the number of hours positive, as it must be. Answering 5 uses only the 10 and forgets the 4 degrees above zero it has to fall through first; 3 comes from 10 − 4 = 6 followed by 6 ÷ 2.