David spends #1/4# of his allowance on Monday. This means that he has #3/4# of this allowance left. He then spends #2/3# of the #3/4# allowance that he has on Tuesday. #2/3*3/4=2/4=1/2#, so he spent #1/2# of his allowance on Tuesday. That leaves #1/4# of his allowance left to spend.