Maria is solving this number riddle by using guess and check: Fifteen less than a number is twelve. Based on the riddle, which statements must be true? Check all that apply. Less than means subtraction. Subtract 15 from the unknown number to get 12. 15 minus 12 = 3 The unknown number is 27. 15 + 3 = 18 27 minus 12 = 15 This is an addition problem.