This equation can be solved using simple algebra.
Let's say that the missing is A, hence replacing the box with the A, giving:
35.4 x A = 3.54
We then could divide both sides of the equation- left hand side (35.4 x A) and the right hand side (3.54)- with 35.4, as we would want to get rid of this number from the left to leave us with A only. The equation then becomes:
A = (3.54/35.4)
Simply use the calculator and you'd get 1/10.
Which is C!
