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
