r varies directly as s and t. one set of values is r=28,s=7, t=8. find r when s=12 and t=9

Question

find r when s=12 and t=9

Answer 1

r varies directly s and t then r  = kst

Given r = 28, s = 7, t = 8

28 = k *7*8

28 = k*56

28/56 = k

k = 1/2

Now find r when s=12, t = 9

r = 1/2*12*9

r = 54