Apiece of wire is 25 ft long and cut into pieces of different lengths all pieces that either 3 4 or 5 ft long if you cut two pieces of 5 how many pieces of 3 and 4 ft can you get not leftover wires after cutting

Algebraically you could represent it like this
2*5 + x*3 + y*4 = 25    Multiply 2*5
10 + 3x + 4y = 25        Subtract 10 from both sides.
3x + 4y = 15                Now try this. Let y = 2
3x + 4*2 = 15
3x + 8 = 15                 Subtract 8 from both sides.
3x = 15 - 8
3x = 7                         Ordinarily you would divide by 3 but the question doesn't let you.

7 is a neat number. You could use 4 + 3

So one answer is 
2 fives.
3 Fours
1 Three

You could also do
2 fives
5 threes. Nothing says you must have a four.

You can also do.
3 fives
1 four
2 threes There is nothing to say you can't have another 5

Maybe the answer is 20mm. all i really did was just multiply, which is obvious.