Walter has 124 bills consisting of 5 dollar bills and 10 dollar bills. If his total amount of money is $840, how many bills of each does he have?

For this question here are the formulas,

Let T be the number of 10's

Let F be the number 5's

Let us find the 10's first by this formula.

5 (124 - T) + 10T = 840

620-5T + 10T = 840

then

5T = 220

5 5

T=44

Now let's find the F using this formula.

5F + 10T = 840

Sustitute T

5F + 10 (44) = 840

5F = 400

5 5

F = 80

to Check

F + T = 124

80 + 44 = 124