Question 50474
Bike =  Scooter + $100
Scooter = Skateboard + $60
Skateboard = unknown cost = x
Total cost of all three = $310
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Bike + Scooter + Skateboard = $310 [Basic addition formula]
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(Scooter + 100) + (Skateboard + 60) + (Skateboard) = 310 [Plug-in the values]
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[(Skateboard + 60) + 100) + (Skateboard + 60) + (Skateboard)] = 310 
[Substitute the value of the scooter for "skateboard + 60"]
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((x + 60) + 100) + (x + 60) + (x) = 310 [Substitute "x" for "skateboard"]
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x + 60 + 100 + x + 60 + x = 310 [Remove the parentheis and simplify (add)]
3x + 220 = 310 [Here is the simplified equation of the problem]
3x + 220 - 220 = 310 - 220 [solve for x]
3x= 90
3x/3=90/3
x=30
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Plug-in the values of the scooter to find the value of the other items:
Bike =  Scooter + $100
Scooter = $30 + $60 = $90
Skateboard = x = $30 
Total cost of all three = $310
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Bike =  $90 + $100 = $190
Scooter = $30 + $60 = $90
Skateboard = x = $30 
Total cost of all three = $310
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Check by plugging in all of the values back into the basic formula of addition
190+90+30=310
310=310 [Checks out]