if statement - SML - Function calculates incorrectly -

for course @ university have learn sml. learnd java befor , having problems sml now. have function should calculates entryfee zoo.

fun calcentryfee (erm:bool,dauer:int,dschungel:bool,gebtag:bool):real= let     val c = 7.0 in     if erm c + 14.50 else c + 19.50;     if dauer < 120 c - 4.0 else c;     if dschungel c + 1.5 else c;     if gebtag c / 2.0 else c end; 

the problem function 'returns' 7.0 or 3.5. doesn't seem execute other 3 if-statements.

there no statements in ml, expressions. a;b expression, evaluates a , b , result result of b. consequently, result of first 3 if-expressions thrown away.

also, variables variables in true math sense, immutable. think of program mathematical formula.

what want write following:

fun calcentryfee (erm : bool, dauer : int, dschungel : bool, gebtag : bool) : real = let     val fee =         7.0         + (if erm 14.50 else 19.50)         - (if dauer < 120 4.0 else 0.0)         + (if dschungel 1.5 else 0.0) in     if gebtag fee / 2.0 else fee end