The speed growth given by the problem is 4.2 cm/day, so we must convert each unit to the one asked by the problem. Each centimeter has 10 mm, so 4.2(10)=42 mm. Each day has 24 hours, each hour has 60 minutes, and each minute has 60 seconds, so 24(60)(60)=86400 sec. If we divide 42 mm between 86400 seg we obtain a growth speed of 0,000486 mm/sec.