Alex spent 25% first, so he has now 75%. Then, he spent another 25% of that remaining money, so 25% of 75% is 56.25%. He has $9 left, which means $9 is 56.25% of the money he began with.
Set up a proportion:
9/x = 56.25/100 (9 dollars out of starting money equals to 56.25 per 100)
cross multiply 9 and 100, 56.25 and x
900=56.25x
Isolate the variable by dividing each side by 56.25
x=$16
