8. z varies jointly as x and y. If z=3 when x=3 and y=15 , find z when x=6 and y=9. a. z= 8/5 b. 18/5 c.5 d.8 9. The pressure P of a gas varies directly as its temperature t and inversely as its volume V. What is the equation of variation? a. P=ktV b. V=ktP c. V= kt/P d. P= kt/V