Hi! So what really helps me sometimes is to draw a circle and split it. I shaded 1/4 (Lionel's quarter) first. Now, there is 3/4 of the cake left. Helen has half of the rest, which means she has half of 3/4. 1/2 of 3/4 (which is basically 1/2 x 3/4) = 3/8. This means Helen has 3/8 of the cake. I then split my circle into eighths and shaded in 3. This left me with 3/8 of the cake left for Sophie. This means the fraction that remains for her is 3/8.
