Setting up equations to solve worded problems #1

I have like a few really basic but idontknow how to solve questions. hope someone can help! thanks in advance! ^^

4) Initially 2 particles A and B are 480cm apart. Particles A and B then move towards each other at speed of 15cm/s and 25cm/s. Find the number of seconds that had elapsed before collision takes place

5) The sum of 3 of the exterior angles of a 7-sided polygon is 200 deg. Find

i) the sum of the three interior angles corresponding to the three exterior angles

ii) The sum of the remaining four interior angles

6) A group of pupils is practising their march past.

If they march in pairs, one pupil is without a partner.

If they march in 3s, 5s, or 7s, there is one pupil short.

Calculate the smallest possible number of pupils in this group.

Thankssss!(Rofl)