To solve the equations:

1) 34.2 - (17.9 - a) = 22
First, simplify the equation by solving the inner parentheses:
34.2 - 17.9 + a = 22
16.3 + a = 22
Next, isolate the variable by subtracting 16.3 from both sides:
a = 5.7

2) r + 16.23 - 15.8 = 7.1
Simplify the equation by combining like terms:
r + 0.43 = 7.1
Next, isolate the variable by subtracting 0.43 from both sides:
r = 6.67

3) 16.5 - (t + 3.4) = 4.9
First, simplify the equation by solving the inner parentheses:
16.5 - t - 3.4 = 4.9
13.1 - t = 4.9
Next, isolate the variable by subtracting 13.1 from both sides:
-t = -8.2
Finally, divide by -1 to solve for t:
t = 8.2

Therefore, the solutions are:
a = 5.7
r = 6.67
t = 8.2