# Lesson 6: functions
def gst(amount, rate=0.18):
    """Return GST on an amount (default 18%), rounded to paise."""
    return round(amount * rate, 2)

def fy_label(year, month):
    start = year - (month < 4)
    return f"FY {start}-{str(start + 1)[2:]}"

def grade(marks):
    if marks >= 90: return "A+"
    if marks >= 75: return "A"
    if marks >= 60: return "B"
    if marks >= 40: return "C"
    return "F"

print(gst(1000), gst(1000, 0.12))
print(fy_label(2026, 10), fy_label(2027, 2))
print([grade(m) for m in (95, 80, 61, 40, 12)])