Answer:
(a)10x+5y=50
(b)4 $5 bills
(c)3 $10 bills
Step-by-step explanation:
Part A
Given:
The number of $10 bills = x
The number of $5 bills = y
If the total amount of money in his wallet is $50, then:

Part B
If the number of $10 bills, x = 3
From the equation above:

Therefore, if there are 3 $10 bills, there are 4 $5 bills.
Part C
If the number of $5 bills, y=6, then:

Therefore, if there are 6 $5 bills, there are 3 $10 bills.