Question 877614
There is 1 seat per bike/tricycle so  b+t = 30 ----> t = 30 - b


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You have 70 wheels, which means


2b + 3t = 70


2b + 3(30 - b) = 70 ... plug in t = 30 - b


2b + 90 - 3b = 70


-b + 90 = 70


-b = 70 - 90


-b = -20


b = 20


There are 20 bikes.


To find the number of tricycles, plug b = 20 into t = 30 - b