如何在Prolog規則語句中添加2個變量？

Question

這是事實陳述和規則：

%Fact statement 
fare(asiawing,kch_brisbane,adult,330). 
fare(asiawing,kch_brisbane,child,180). 
fare(asiawing,kl_paris,adult,690). 
fare(asiawing,kl_paris,child,550). 

%Rule statement 
totFareAsiaWing(T,D,C) :- 
    fare(asiawing,D,C,F), 
    T is F*1.45. 

totAmountAsiaWing(Z,D) :- 
    ( write('num of adult passenger :'), 
     read(X), 
     number(X), 
     totFareAsiaWing(T,D,adult), 
     Z1 is X*T 
    ; write('num of child passenger :'), 
     read(Y), 
     number(Y), 
     totFareAsiaWing(T,D,child), 
     Z2 is Z1+Y*T, 
     write(Z2) 
    ). 

如何添加Y*TZ爲X*T和Z？我想補充，但它給了我

ERROR: is/2: Arguments are not sufficiently instantiated