Use selected index value to retrieve value in array

I have a multi dimensional array as follows:

arr[0] = [500,40,50,10,7.5];
arr[1] = [1000,40,80,10,12];
etc

and the value of a selected index, x
I want to compare x to the array in such a way that allows me to find the 3rd value in the row that corresponds to the value of the selected index

So, it x =1 I want to retrieve the value 80