David spends 1/414 of his allowance on Monday. This means that he has 3/434 of this allowance left. He then spends 2/323 of the 3/434 allowance that he has on Tuesday. 2/3*3/4=2/4=1/223⋅34=24=12, so he spent 1/212 of his allowance on Tuesday. That leaves 1/414 of his allowance left to spend.