b)
[tex]90 = 2 \times {3}^{2} \times 5[/tex]
[tex]168 = {2}^{3} \times 3 \times 7[/tex]
[tex](90,168) = 2 \times 3 = 6[/tex]
c)
[tex]100 = {2}^{2} \times {5}^{2} [/tex]
[tex]280 = {2}^{3} \times 5 \times 7[/tex]
[tex]980 = {2}^{2} \times 5 \times {7}^{2} [/tex]
[tex](100,280,980) = {2}^{2} \times 5 = 20[/tex]
d)
[tex]875 = {5}^{3} \times 7[/tex]
[tex]550 = 2 \times {5}^{2} \times 11[/tex]
[tex](875,550) = {5}^{2} = 25[/tex]
