Question

Find a set of 4 distinct positive integers a,b,c,d such that the smallest positive integer that can not be represented by such expressions involving a,b,c,d (instead of 1,2,3,4) is greater than 22.You can use digits exactly once. You are allowed to reuse operators (*, +, -). You are not allowed to join digits together (12+34). / (divide) is not one of the allowed operators.