Answer:
he needs 20.25 bags of sand and it would cost her 80.79
Explanation:
150cm*90cm*15cm=202,500cm^3 this is how much sand she needs to fill it
202,500cm^3/10,000cm^3=20.25 this is how many bags he needs
3.99*20.25=80.79 this is how much it'll cost him for all 20.25 bags