Question 389296
On Monday, Carlos drove to work in 35 minutes.
 On Tuesday, he averaged 9 miles per hour more and arrived 7 minutes faster.
 How far (in miles) does Carlos travel from home to work? 
:
Let d = distance traveled to work
Let s = average speed on monday
then
(s+9) = average speed on tuesday
:
Write two time equations: time = dist/speed
{{{d/s}}} = {{{35/60}}}
Cross multiply
60d = 35s
Simplify, divide by 5
12d = 7s
12d - 7s = 0
and
{{{d/((s+9))}}} = {{{28/60}}}; (7min faster)
Cross multiply 
60d = 28(s+9)
60d = 28s + 252
Simplify, divide by 4
15d = 7s + 63
15d - 7s = 63
:
Subtract the 12d - 7s = 0 from the above equation
15d - 7s = 63
12d - 7s = 0
----------------subtraction eliminates s, find d
3d = 63
d = {{{63/3}}}
d = 21 miles is C's distance to work
:
:
Check this: find the the speed
7s = 12(21)
7s = 252
s = 252/7
s = 36 mph
:
Find the times
21/36 = .583 hrs
21/45 = .467 hrs; (9 mph faster)
-----------------
difer =  .116 hrs which is about 7 min