What are some methods you can use to multiply a whole number and a decimal

At which value in the domain does f(x) =0?

Which one is the correct answer, and why?

Will mark ! (10 points also! ) if x varies inversely with y and x =6 when equals y= 10, find y when x=8 a. y=4.8 b. y=7.5 c. y=40/3 d. y=4/15