Step-by-step explanation:
x = number of $50 bills
y = number of $100 bills
x + y = 20
50x + 100y + 500 = 100x + 50y
50y + 500 = 50x
y + 10 = x
now we use that in the first equation :
y + 10 + y = 20
2y = 10
y = 5
x = y + 10 = 5 + 10 = 15
so, he has 15 $50 bills and 5 $100 bills = $750 + $500 = $1250 in his wallet