Question

Dora drove east at a constant rate of 75 kph. One hour later, Tim started driving on the same road at a constant rate of 90 kph. For how long was Tim driving, before he caught up to Dora? 
I have tried to use a table
i set it up like this
Name|D(mi)|R(kph)|T(hr.)|
Dora  | d     |75      |t      |
Tim   |d       |90      |t+1  |
and then I went and i tried to put it into an equation
75t=90(t+2)
75t=90t+180
-90t -90t
-15t=180
____ ____
-15 -15
t=-12
but It is not an answer on my multiple choice and it wouldn't be a negative 12 hours

Answer 1

Dora = 75 kph.

Tim = 90 kph,  One hour later.

If Dora has been driving for t hours, then Tim would have driven for (t-1).  Since he started his own trip one hour later.

Therefore the both distances covered would be equal at the meeting point.

75t = 90(t-1)
75t = 90t -90
75t-90t = -90
-15t = -90 .    Divide both sides by -15.

t = -90/-15 = 6

Recall, Tim was driving (t-1)  = (6-1) = 5
Tim was driving for 5 hours before he caught with Dora.
Cheers.

Answer 2

Ok so let T=time from when Tim starts
Distance Tim travels= T*90
Distance Dora travels=T*75 + 75  (<as already started)
for him to have caught up His distance must equal hers so:
90T=75T + 75  (now minus 75T)
15T=75  (now divide by 15)
T=5 :)
So five hourse, hope this helped :)