i have no idea how to get the answers

please help

1. do the points P(1,-3), Q(2,1) and R(2.5,3) lie on the same straight line.

2, An electronic bankteller registered $775 after it had counted 120 notes and $975 after it had counted 160 notes.

a) find the formula for the sum registered ($C) in terms of the numbe rof notes (n) counted

3.

Da=10t+20

Db=20t+5, where t is the time in hours after 1.00pm

a) find the time in hours at which the two cyclists are at thesame distance from the town hall step

attempt.

10t+20=20t+5

-10t=-15

t=15/10 ? but the answer is 2.00pm

thanks alot