What would someones mass be if their weight was 158.4? this is for science homework, they gave me a random weight # but no mass.

ON EARTH, every kilogram of mass weighs about 2.205 pounds.

So if you know the weight (in pounds), and you know that it was
weighed on Earth, you can simply divide the pounds by 2.205 ,
and you'll get the mass (in kilograms).

I can see that on this science homework, they used 2.2 for the weight
of 1 kilogram expressed in pounds.  So just divide the weight by 2.2
to get the mass in kilograms.

(The weight number they gave you is not so random.  :-)  )