1.Why were you late for school today?
2.Please wipe these 39 desks.
3.There are lots of posters on the platfprm,about 40 pieces.
4.There are 39 students in our class,22 of which are girls,17 of which are boys.
5.I have 40 pens ,15 of which are blue,14 of which are black,11 of which are red.