The P6 syllabus is now teaching algebra albeit in a slightly
modified way than what you learn in secondary school.
They use rectangular blocks instead of alphabets. 
=P fine then. let x be the distance of the road divider. x/3-x/5
= 20 ==> x = 150 m. number of trees should be 82. draw
your own model. =P
you could also just find the HCF of 5 and 3, find the difference
in number of threes between them, then find the distance you need
to make a difference of 20 trees. this is probably the way that you
are required to do.
lazy to think of the 3rd one, let's see if anyone else
answers.