$$18+54 = 72$$  $$90-72 = 18$$  $$18\div2=9$$ 
Andy is $18$ years old and his mother is $$54$$ years old now. In how many years\textquotesingle{} time will the sum of their ages be $$90$$? 
a: $$9$$ 
b: $$14$$ 
c: $$18$$ 
d: $$28$$ 