Posted by Dan on Friday, December 17, 2010 at 10:16pm.
to identical jars are filled with equal amount of marbles. the marbles are coloured red or white. the ratio of red to white marblesis 7:1 in jar 1 and 9:1 in jar 2. if there are 90 white marbles altogether, determine the number of red marbles in jar 2.

Math  anon, Friday, December 17, 2010 at 10:34pm
In jar 1, 1/8 of the marbles are white.
In jar 2, 1/10 of the marbles are white.
Since the jars are filled with an equal amount of marbles, take this value to be x. Therefore, x/8 + x/10 should equal to 90, the total number of white marbles.
From this, we get x = 400 marbles altogether.
This means 200 marbles in each jar.
In jar 2, 9/10 of the marbles are red. There are 200 marbles in this jar. (9/10)*200 = 180 red marbles in jar 2. 
Math  Dan, Friday, December 17, 2010 at 10:56pm
how did you get x to equal 400

Math  anon, Tuesday, December 21, 2010 at 7:00pm
By solving the equation x/8 + x/10 = 90.
Sorry for the late response.