#### Answer

(answers vary)
The main reason in not getting $1$ as an answer is that you made a mistake in the addition of the fractions.
Some possible mistakes are:
(1) You made a mistake in converting some fractions to a fraction whose denominator is 20 (the LCD).
(2) After making the fractions similar using their LCD of 20, you made a mistake in adding the numerators.
(3) You made a mistake of adding the numerators together and adding the denominators together to get $\frac{7}{29}$.

#### Work Step by Step

In Exercise 21, if you added the fractions for the groups and did not get $1$ as answer, then you must have made a mistake in the addition.
Some possible reasons are:
(1) You made a mistake in converting some fractions to a fraction whose denominator is 20 (the LCD);
(2) After making the fractions similar using their LCD of 20, you made a mistake in adding the numerators; or
(3) You made a mistake of adding the numerators together and adding the denominators together to get $\frac{7}{29}$.