It appears that the distribution in question may be a continuous uniform distribution. In that case there is an infinite number of possible values between 45 and 150. Therefore the probability that x is exactly 48 is zero.
Ever 2.2 lbs is 1kg, so to find how many kg in 50 lb you would divide 50 by 2.2. 50lbs is equivalent to 22.72 kg.
I need a little more information is there anymore that you could give me?