The probability of hitting equally distanced mines on a straight line Recently I was given the following problem.

Anti-tank mines are placed on a straight line 15 meters apart from each other. The tank, 3 meters wide, runs perpendicular to this line. What is the probability that the tank will hit a mine.

My problem with this question is that the exact length of the line is not given. I guess depending on the length of the line, the probability might be different. It seems to me that the problem is not well defined. What is your opinion?
 A: The following are implicitly assumed:


*

*the row of mines is infinite, with mines 15 metres apart

*the tank's position is continuously distributed


Then we can restrict the scope of the tank so its left edge runs between two adjacent mines; it never detonates the left mine. Say the tank is moving forward to an east-west row of mines; if and only if the left edge is less than 3 metres from the right mine will it detonate the right mine. The probability is thus $3/15=0.2$.
A: The distance of 3 meters to each side of a mine would equal to 6 for each mine.  The total distance is 15 between each mine.  The tank would be blown away within this 6 meters distance around each mine and hence the probability is $\frac{6}{15} = .4$
A: The true tank doesn't crawl, dragging its bottom over the ground; it rather rides on wheels, which roll over two caterpillar tracks. So it's actually ill-posed (or rather falsely modelled) situation, you would need to know the track width together with the distance between tracks, not the whole vehicle's width.  
However, assuming the simplified problem, you may consider the position of the tank's centre over the mine line. If the centre falls inside a 3-meter segment centered over any mine, the tank gets destroyed, otherwise it crosses the line untouched. There is one 3-meter death zone per every 15-meter segment between mines, so the probability of hitting a mine is $3/15 = 1/5$.
