Marj has x $20 bills, so the total amount of money from the $20 bills is 20x.
She also has three $5 bills, so the total amount of money from the $5 bills is 3*5 = 15.
She has $4.50 in loose coins.
The total amount of money in her wallet is the sum of the amount from the $20 bills, the amount from the $5 bills, and the amount from the loose coins: 20x + 15 + 4.50 = 20x + 19.50.
She buys a cake that costs $17.50, so the amount of money left in her wallet is: (20x + 19.50) - 17.50 = 20x + 2.
We are given that the amount of money left in her wallet is $42, so we can write: 20x + 2 = 42.
Subtracting 2 from both sides, we get: 20x = 40.
Dividing both sides by 20, we get: x = 2.
The value of x is 2.
#### 2
The answer is: 2
Below is a math problem. Provide a detailed, step-by-step solution.

### Problem:
Marj has x $20 bills, three $5 bills, and $4.50 in loose coins in her wallet. If she buys a cake that costs $17.50, how much money will be left in her wallet?
If we know the answer to the above question is 42, what is the value of unknown variable x?

### Answer:
