-- replace "undefined" by your definition

volume r = undefined

heliumCosts r = undefined

balloonRadius m = undefined


-- the following tests can be used by you, once you have implemented your functions,
-- for testing, just invoke testV, testHC1, testHC2, testBR1, testBR2 in ghci

testV = "test volume: expected ~ 1436.755, computed: " ++ show (volume 7 :: Double)
testHC1 = "test heliumCosts 1: expected ~ 4.27, computed: " ++ show (heliumCosts 20 :: Double)
testHC2 = "test heliumCosts 2: expected ~ 34.17, computed: " ++ show (heliumCosts 40 :: Double)
testBR1 = "test ballonRadius 1: expected ~ 21.08, computed: " ++ show (balloonRadius 5 :: Double)
testBR2 = "test ballonRadius 2: expected ~ 26.56, computed: " ++ show (balloonRadius 10 :: Double)