<p>^ Substitution only works unto a certain degree. What if they gave fractions such as 2/23, Then 3/91? </p>
<p>They did this twist in the SAT Practice question 3 in tip 4. </p>
<p>Rather then one denominator in all four boxes, there were different denominators. So substitution would have took 40548290 minutes to do. Chungs way of doing it was much more faster and more accurate/less chance of messing up.</p>