ช่วยอธิบายข้อนี้ให้หน่อยค้า

Rule 7. Use a singular verb with distances, periods of time, sums of money, etc., when considered as a unit.

Examples:
Three miles is too far to walk.
Five years is the maximum sentence for that offense.
Ten dollars is a high price to pay.
BUT
Ten dollars (i.e., dollar bills) were scattered on the floor.