The first step here is to write out this question as a proper algebraic expression. That means you assign a letter to the question mark and you now have;
4/6 = y * 1/6
(Note that the symbol * represents the multiplication sign)
y * 1/6 can be re-written as y/6 since it is actually y/1 * 1/6
Therefore you now have;
4/6 = y/6
Cross multiply and you now have,
4 * 6 = y * 6
24 = 6y
Divide both sides of the equation by 6 (to eliminate the 6 on the right hand side)
4 = y
Therefore the number that should be in the question mark is 4.