Each pair of numbers (except for the last one) multiplies to form the next one or two numbers in the chain.<br>
6*7=42<br>
7*4=28<br>
4*2=8<br>
2*8=16<br>
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The longest number that follows this pattern is 6212248324.<br>
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StevenRoy solves this with <a href="http://michrev.nfshost.com/t/multiplydigitstringthing.html">a computer search</a>.