John has an empty box. he puts some red counters and some blue counters into the box. the ratio of the number of red counters to the number to the number of blue counters is 1: 4 linda takes at random 2 counters from the box. the probability that she takes 2 red counters is 6/155. how many red counters did john put into the box?

R/B = 1/4 
R = 4B 
For every red, there are 4 blues 
Let there be x reds and 4x blues (5x counters in all) 
P(red,red) = (x/5x)((x-1)/(5x-1)) = 6/155 
(155)x(x-1) = (6)5x(5x-1) 
155x^2-155x = 150x^2-30x 
5x^2-125x = 0 
5x=125 
x= 25 reds

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