array.map(&:cash).inject(0, &:+)
or
array.inject(0){|sum,e| sum + e.cash }
In Ruby On Rails you might also try:
array.sum(&:cash)
Its a shortcut for the inject business and seems more readable to me.http://api.rubyonrails.org/classes/Enumerable.html
#reduce takes a block (the &:+ is a shortcut to create a proc/block that does +). This is one way of doing what you want:
#reduce
&:+
+
array.reduce(0) { |sum, obj| sum + obj.cash }