You working on string '90%' and trying to complete math operation, which will fall. This should work:
var inside_element = $('figure');
var indx = 10;
inside_element.each(function(indx){
$(this).css({
width: (90 - (10*indx)) + '%' ,
height: (90 - (10*indx)) + '%'
});
});
Also declaration var indx = 10;
is not necessary. This value will be overrided inside function.
EDIT: Also there can be more containers. Then code should look like this:
var inside_container = $('.inside_container');
inside_container.each( function(i) {
var inside_element = $(this).find('figure');
var step = 10;
inside_element.each(function(indx){
$(this).css({
width: (90 - (step*indx)) + '%' ,
height: (90 - (step*indx)) + '%'
});
});
});