Question

if x varies jointly as y and z, and x=10 when y=4 and z=15, find x when y=8 and z=20

Answer 1

do you have more points? because it could be many things from that setup
x could be 13
y: 4->8 = +4
z: 15->20 +5
so
x: 10->13 +3
just a guess

Answer 2

I'm afraid those are the only points i was given

Answer 3

So x varies jointly as y and z would be
x=k(yz)
Plugging in x=10, y=4, z=15
10=k(4)(15)
Now solve for k

Answer 4

so k would = .16?

Answer 5

Not quite.
k=1/6
10=60k. Divide 60
So k=1/6

Answer 6

So you have x=1/6 (yz)
And you have y=8 and z=20
And you're trying to solve x. So plug the numbers in

x=1/6 (8)(20)
So what's x?