Acashier collects 124 bills totaling $920 dollars. assuming that the cashier collects $5, $10, and $20 bills, and that she collects twice as many fives as tens, how many tens does she collect?

Let the no. of $10 bills be x, the no. of $5 bills be 2x, and the no. of $20 bills be y.

A cashier collects 124 bills => x + 2x + y = 124  => 3x +y =124

totaling $920 dollars => ($10)(x) + ($5)(2x) + ($20)(y) = $920 => 20x + 20y = 920

Now you got 2 equations :
3x +y =124 &
20x + 20y = 920

3x +y =124 => y=124-3x
Then Sub y=124-3x into 20x + 20y = 920,
ie. 20x + 20(124-3x) = 920 
     20x + 2480 - 60x = 920
     2480 - 40x = 920
      2480 - 920 = 40x
               1560 = 40x
                      x = 39

The additive identity is 0 because when you add 0 to a number, you get the identical number as an answer.a + 0 = a