Let the original length be x, (x- 18)=4(x-39).

Solve the unknown x; Day 46

If you don't learn the unknowns, it's reverse thinking: treat the remaining first bundle as a whole, which is four times that of the remaining second bundle, then if you divide the first bundle into four parts, each part should be equal to the second bundle. Then 39- 18 is three copies, and the remaining first copy is (39- 18)/3*4=28.

Then add 46 of 18.