Now when I reduce fractions while multiplying I am getting confused. I know I can cross cancel AND do vertical reduction.

Now when i do the problem:

18/12 ÷ 35/70
I reduce to
3/2 ÷ 1/2


I then FLIP...cross cancel and multiply like so:


3/2 • 2/1
Cross cancel the 2's
3/1 • 1/1 = 3/1



This problem is correct. However. When I do the same thing with the following problem, I get an incorrect answer. What am I doing wrong?


240/10 ÷ 65/100
I reduced to
24/1 ÷ 13/20


I then...cross canceled 20 and 24.....flipped...and multiply like so:


6/1 • 13/5
Flip and multiply
6/1 5/13 = 30/13





that is the wrong answer. it should be 36 and 12 over 13.

What did I do wrong and why is it wrong?

You cannot "cross/cancel" over division....only multiplication.

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Oh duh, thanks a bunch.

You haven't "flipped" yet! You can't cancel numbers in the two different fractions until after you have flipped them
24/1 ÷ 13/20= 24/1 x 20/13 and now the 24= 4(6) and 20= 4(5) are both in the numerator- they cannot be canceled.