Set up equations:
(1) speedbus = speedbike+4
(2) speedbus/speedbike = 180/156
plug (2) into (1)
speedbike * 180/156 = speedbike + 4
(solve for speedbike)
speedbike = 4 / (180/156-1) = 26 mph
--> speedbus = 26+4 = 30mph
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