Expert: Steve Holleran - 8/26/2007

Question
If y varies jointly as x:z and 9x:5=3y:8z , find the value of y when x=10 and z=2. How do I answer this question? Sorry if this is simple but I'm from Singapore and I'm just 14.

Answer
Hi Adisen,

Hey, you're doing great if you're only 14 and working on this material!!

It's not simple, but its also not as difficult as it sounds.  The key part of the problem is the section which gives you:

               9x / 5 = 3y / 8z  (You want to set this up
                                 as fractions, not with
                                 the : sign)

then multiply by (8z):

              (9x)(8z) / 5 = 3y

Now divide by 3 to isolate y:

              y = (9x)(8z) / 15 = (72xz) / 15

Now substitute the values:

              y = (72 * 10 * 2) / 15 = 1440 / 15 = 96

I think what confuses some students is the first phrase.  It just sets up what type of problem it is.  The stuff in the middle is the important stuff.

Hope you understood my steps OK.  Good luck.

Steve Holleran