The key answer of exercise Grammar 3
The key to exercise solutions in chapter 04 - Language focus 1 for the textbook Cutting Edge Upper-Intermediate - Student's Book with authors Sarah Cunningham, Peter Moor and Jonathan Bygrave from Pearson Education
Question
Relative clauses with passives can be shortened. Look at the example. Then find a similar example in exercise 1a and shorten it in the same way.
Answer
1 ... the mathematical skills that are required are not ...