A cashier 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