Shaun plotted a point on the number line by drawing 5 equally spaced marks between 0 and 1 and placing a point on the third mark he claims that point represents the fraction 3/5because each mark represents 1/5 so the third mark represents 3/5

Shaun is correct, he can use 5 as the denominator of the fraction or partition of line with measurement of 1 unit (the point is in between 0 to 1). It is also mentioned that the point has been placed in equal distance or equally marked. Then, the third point is 3/5 and the first point is 1/5.