<human>:  In the jar, there are 7 blue marbles, 11 red marbles, and some yellow marbles. If the chance of randomly picking a yellow marble is 1/4, how many yellow marbles are there?
<bot>: Define a variable ** Let y be the number of yellow marbles
How many total marbles are there? ** There are 11+7+y=18+y total marbles.
Write an equation ** y/(18+y)=1/4.
Cross multiply ** 4y=18+y
Subtract y ** 3y=18
Divide by 3 ** y=6
How many yellow marbles are there? ** There are 6 yellow marbles.
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