From the condition,    marbles are blue.


Hence,   of the marbles are 18 blue marbles.


It means, that   of the marbles is   = 6 marbles.


Therefore, the number of the blue marbles is 6*7 = 42.


Altogether, Ben has 18 + 42 = 60 marbles.    ANSWER

With  being red,  is blue.

We then get the following PROPORTION, with T being the total number of marbles:      

 ------ Cross-multiplying

3T = 7(18) ----- Cross-multiplying

T, or total number of marbles =  

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