Jamel watched his older brother robert lift weights. the bar alone had a mass of 20 kg. on the bar he had tow 11.4 kg weights, two 4.5 kg weights , and four 450 g weights. what mass was robert lifting ?

All you do is add them all together...
20 + 11.4 + 11.4 + 4.5 + 4.5 + 0.45 + 0.45 + 0.45 + 0.45 = 53.6Kg
(note that 0.45 is 450g in Kg, When adding things such as weight or length you want to make sure that they are all in the same unit)