Let

x = the number of Php20-pesos bills, x>=0, x is integer

y = the number of Php-50-peso bills, y>=0, y is integer

20x + 50y = 500 divide both sides by 10

2x + 5y = 50

y = 10 - 0.4x

x = 0, y = 10

x = 5, y = 8

x = 10, y = 6

x = 15, y = 4

x = 20, y = 2

x = 25, y = 0

Php500 cane be made up of both Php20-peso and Php50-peso bills in 4 ways.