area of the field should be 9*10 = 90 square feet.
a bag of grass seeds cover 15 square feet.
so if you need to know how many bags should be used, then,
Total area of field/ area covered by 1 bag of grass seeds.
= 90/15 = 6 bags
he needs to use 6 bags of grass seeds to cover the whole field.
I don't think there are any 'hidden' questions, though you can consider the 2nd part of the question I guess. (how many bags of seed will Paul...)
