To start this problem, we know that one of the three pieces that the string is cut into is 15 meters longer than the others. Subtract 15 from 42 to get 27.
Then, from there, you can cut that number (divide) by three to get the length of the two shorter strings. Simply add the 15 meters you didn't use yet to one of the pieces to find the longer string, and you have your lengths.
Hope this helps!
-refrac532
