Answer:
19 1-dollar bills
Explanation:
We will call the number of 1 dollar bills "a"
We will call the number of 5 dollar bills "b"
a + b = 30 (the number of 1 dollar bills + the number of 5 dollar bills = 30 bills in total)
1a + 5b = 74 ("a" 1 dollar bills are worth 1a dollars, and "b" 5 dollar bills are worth 5b dollars. In total they are worth 74 dollars)
Next, you can "solve for a variable" in the first equation to plug into the second equation
b = 30 - a (move "a" to the other side, now "b" is equal to 30 - a)
1a + 5(30-a) = 74 (substitute "b" in the second equation for the new value of "b" you found in the previous step)
1a + 150 - 5a = 74 (distribute out 5(30-a) to get 150 - 5a)
-4a = 74 - 150 (combine like terms)
-4a = -76 --> a = 19 (simplify the right side, and divide both sides by -4 to get answer)
Refer back to the first sentence of the explanation where we said the variable "a" equals the number of $1 bills there are. Therefore, there are 19 $1 bills.