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