I dont get how to calculate the time with multiplexors decod

Solution

The critical path is defined as the path from input to output which has the maximum delay. You have to analyze the path from input to output which has the largest delay.

1. AND (3ns) + OR(4ns) + OR(4ns) + AND(3ns) = 14ns

2. NOT(1ns) + AND(3ns) + OR(4ns) +OR(4ns) = 12ns

3. AND(3ns) + OR(4ns) + NAND(2ns) + OR(4ns) = 13ns

4. NOT(1ns) + NOR(2ns) + OR(4ns) + AND(3ns) + OR(4ns) = 14ns

If any multiplexer, decoder or encoder comes in your critical path than you have to consider its timings.