Samuel has $5, $10, and $20 bills which amount to $150 in his wallet. His $10-bills amount to $50, and he has 4 $20-bills. How many bills does Samuel have in his wallet? Let's be accurate as possible.
His $20 bills amount to 4 x $20 = $80. So the total amount of the $10 and $20 bills is $80 + $50 = $130. Hence, the remaining $5 bills amount to $150 - $130 = $20. This means that there are $20 / $5 = 4 $5 bills. There are also $50 / $10 = 5 $10 bills. Therefore, Samuel has 4 + 4 + 5 = 13 bills in his wallet.
The answer: 13.