solve variation equation

Question

The quantity q varies jointly with s and t and inversely with r when s=-10, t=-1, and r=-4, q=25. Find q when s=7, t=4, and r=-4

Answer 1

q = kst/r,

 

where k is a constant.

 

k = rq/(st) = (-4)(25)/[-10(-1)] = -10

 

q = -10st/r

 

For

 

s = 7

t = 4

r = -4,

 

q = -10(7)(4)/(-4) = 70