Josh starts with 8/8 (or any other fraction that equals 1) of his allowance. He then spends 3/8 of it.
8/8 x 3/8 = 3/8
8/8 - 3/8 = 5/8
He spend 3/8 of his allowance, leaving him with 5/8 left. Then Josh spent 2/3 of that remaining amount.
5/8 x 2/3 = 10/24 = 5/12
Josh has 5/12 of his allowance left for the rest of the week.
Hope this helps!