In a 100 m race, A can give B 10 m and C 28 m. In the same race B can give C:

A : B = 100 : 90.

A : C = 100 : 72.

B:C = B/A*A/C = 90/100*100/72 = 90/72

When B runs 90 m, C runs 72 m.

When B runs 100 m, C runs (72/90*100)m = 80m

 B can give C 20 m.