Question

If f(x) varies directly with x and f(x) = 2 when x = 10, then what is the value of f(x) when x = 40?

a.8
b.45
c.200
d.320

Answer 1

this is simple multiplication
do 2*4 and 10*4
so you get f(x)=8 and x=40

if you wanna do it by doing division do
2=10
f(x)=40
cross multiply then divide
so do 2*40 to get 80
then do 80/10 to get 8

Answer 2

Answer: a.8

Explanation:

Given : If f(x) varies directly with x and f(x) = 2 when x = 10

We know if two quantities (x and f(x)) are directly proportional then

`(x_1)/(f(x_1))=(x_2)/(f(x_2))`

Now, for the given problem we have ,

`(10)/(2)=(40)/(f(40))\\\\\Rightarrow\ f(40)=(40*2)/(10)\\\\\Rightarrow\ f(40)=8`

Hence, if x = 40, then f(x)=8.