a) A,Al=27 ---> 1mol atom Al=27g= 6.023×10^23
27g.................6.023×10^23
2.7g...................x
x=2.7×6.023×10^23/27=6.023×10^22 at. Al
b)40gCa.............6.023×10^23
4gCa...............y
Y=4×6.023×10^23/40=6.023×10^22 at. Ca
c) 24gMg...............6.023×10^23
1gMg...................z
z=1×6.023×10^23/24=0.25096×10^23 at. Mg