Hair grows rate

So lets do it by a simple basic way. (with Distance=Time*Speed formula) We know D = 20 cm and S= $4*{ 10 }^{ -9 }$ m/s First we have to convert 20 cm to m (according to the S units). So, D = 20/100m = 1/5 m or 0.20 m Now Use the formula T=D/S T = 0.20 * 1000000000/4 after solving we get T = 50000000 s or $5*{ 10 }^{ 7 }$ s Now convert seconds into months (sounds crazy but it is easy to do) We know there are 30 days in a month, 24 hours in a day, 60 minutes in an hour and 60 seconds in a minutes. So approx. seconds in a month will be 30 * 24 *60 * 60 seconds. Now divide $5*{ 10 }^{ 7 }$ s by 30 *24 * 60 * 60 s and then you will get approx. 19.29 , which is eventually equals to 19 months . If you found the solution satisfying and easy, then nothing, just enjoy :p. HAVE A NICE DAY.

In one second hair grows by 4 x 10E-9 m, 20cm = 2 x 10E-2. 2X10E-2/ 4X 10E-9 = 5X10E7 = 50,000,000. Thus it takes 50,000,000 seconds for hair to grow by the required amount. One month can be taken as 30 days, = 30 x 86,400 seconds = 25,920,000 seconds. 50,000,000/ 25,920,000 is approximately equal to 19, so it should take about 19 months for hair to grow by 20cm. Therefore our answer is C) about 19 months.