Answer:
Noah spent 4/8 of his allowance on Monday.
Explanation:
Given that Noah spent some of his allowance on Monday, 1/8 of his allowance on Tuesday, 1/8 of it on Wednesday and 2/8 of it on Thursday, and has none of his allowance money left, to determine what fraction of his allowance did he spend on Monday the following calculation must be performed:
8/8 - (1/8 + 1/8 + 2/8) = X
8/8 - 4/8 = X
4/8 = X
Therefore, Noah spent 4/8 of his allowance on Monday.