1-how many atoms are present in 4.0mol of sodium?
2-convert 3.01 x 1023 atoms of silicon to moles of silicon?

Part 1:    If 1 mol = 6.022 x 10^23 atoms of anything, including Na, then,

4.0 mol Na = 4 x 6.022 x 10^23 atoms Na = 2.4 x 10^24 atoms Na 

Part 2:  3.01E23 / 6.022E23 and the answer comes out to .49983394 you would then round this to .500 with significant figures. Hope I helped. :)