Đáp án + Giải thích các bước giải:
`15.` The meeting will be held before May Day.
`16.` The engine of the car has to be repaired.
`17.` The window was broken and some pictures were taken away by the boys.
`18.` A lot of money is spent on advertising everyday.
`19.` A story is going to be told by the teacher.
`20.` The cake is being cut with a sharp knife by Mary.
