I have also come across this question in a defence force aptitude test

although the original question appeared as follows

If 3x = 81, what is the value of x with the 'x' being a power

the answer is d) 4 because 3x3x3x3= 81

my best advice is to use trial and error in that use each of the possible answers as a substitute for 'x' until you find the correct one

although the original question appeared as follows

If 3x = 81, what is the value of x with the 'x' being a power

the answer is d) 4 because 3x3x3x3= 81

my best advice is to use trial and error in that use each of the possible answers as a substitute for 'x' until you find the correct one