If the force F and the length L are inversely proportional, then their product is constant:
[tex]FL=k[/tex]
Given the values F=20 and L=1.5, we can deduce that the constant is
[tex]20\cdot 1.5=30[/tex]
So, given L=6, we can solve for F as follows:
[tex]6F=30 \iff F=5[/tex]
