First we want to express their work in speed.
For sophie we can write:
x/45 = 1
that means that after 45 minutes (x=45) she will make 45/45 = 1 work (finish those potatoes)
for Simon we write:
x/30 = 1
Now if they work together we just need to sum their speed.
x/45 +x/30 = 1
5x/90 = 1
x = 90/5
x = 18
Answer is b) 18 minutes.
