Wvaries jointly as x and p . if w=78 when x=2 and p=10, find x when w=858 and p=5.5

W=kxp
where
k is the constant of proportionality given by:
k=w/xp
 If w=78 when x=2 and p=10 then
k=78/(2*10)
k=3.9
thus the formula is:
w=3.9xp

b] find x when w=858 and p=5.5
plugging the values in the formula we get:
858=3.9×5.5×x
hence 
x=858/(3.9×5.5)
x=358/21.45
x=16.69