Explanation:
Draw 6 identical rectangles side by side as shown in figure 1 in the diagram below. Shade one of the rectangles to represent 1/6 of the cake left over.
Now draw in horizontal lines (see figure 2) to split the leftover amount into 7 equal portions. Notice that we have a 7 by 6 grid of tiny rectangles. There are 7*6 = 42 tiny rectangles in total.
Each person will therefore get 1/42 of the original cake when splitting up this last leftover slice.
