42 is 14% of what number?

"is" means =

14% = 0.14 as a decimal

"of" means to multiply

"what number" becomes "x"

so you can get

42 = 0.14 * x divide both sides by 0.14

x = 42 / 0.14 = 300

so your answer would be 300

It's 300

if 42 is the 14%, how will the 100%?

Easy:

100 x 42 / 14 = 300