а) 2 9/10 - (1 2/5 + 3/10) + 7/20
First, convert all mixed numbers to improper fractions:

2 9/10 = 29/10
1 2/5 = 7/5

Now we can rewrite the equation:

29/10 - (7/5 + 3/10) + 7/20
= 29/10 - (14/10 + 3/10) + 7/20
= 29/10 - 17/10 + 7/20
= 12/10 + 7/20
= 6/5 + 7/20
= 24/20 + 7/20
= 31/20 or 1 11/20

б) 4 - (41/84 - 5/21) + 7 11/30
First, find a common denominator for the fractions in the parenthesis:

41/84 = 5/12
5/21 = 28/84

Now we can rewrite the equation:

4 - (5/12 - 28/84) + 7 11/30
= 4 - (5/12 - 28/84) + 221/30
= 4 - (5/12 - 28/84) + 221/30
= 4 - (35/84) + 221/30
= 4 - 35/84 + 221/30
= 4 - 35/84 + 221/30
= 120/30 - 35/84 + 221/30
= 4 - 35/84 + 221/30
= 120/30 - 35/84 + 221/30
= 4 - 35/84 + 221/30

Now, find a common denominator and simplify the result.