Three babies A, B and C cry out at every interval of time. Baby A cries out at every 20 minutes, while B
cries out at every 30 minutes and C at every 40
minutes. If they are observed at the same time, after
what time will the three of them cry together?​

First step is to get rid of the square root. remember that a square root is the same as raising to the (1/2) power. see below: multiply exponents per exponent rules to get: remember you subtract exponents from the denominator and add exponents in the numerator so: simplify further: so a is the correct choice. if you don't understand this google exponent rules and it will you understand. look at the image results.