RE: Math mini-contest problem for Day 5 on D.Buzz for March 2021 😎

y = # ₱500 bills
x = # of ₱50 bills = 8y
z = # of ₱200 bills = x - 50 = 8y - 50

50x + 500y + 200z = 15,000
50(8y)+500y+200(8y-50)=15,000
400y+500y+1600y−10000=15000
2500y=25000
y=10
x=8y=80
z=x-50=30 --> 30 ₱200 bills