An application executes 6.4×108 number of instructions in 6.3 seconds. There are four types of instructions, the details of which are given in the table. The duration of a clock cycle in nanoseconds is ______. (rounded off to one decimal place)

Instruction type	Clock cycles required per instructions	Number of Instructions executed
Branch	2	2.25 x 108
Load	5	1.20 x 108
Store	4	1.65 x 108
Arithmetic	3	1.30 x 108

.

The correct answer is 3.

To find the duration of a clock cycle in nanoseconds, we need to calculate the total number of clock cycles executed by the application and relate it to the total execution time.

Step 1: Calculate the total number of clock cycles required
Using the table provided in the question, we determine the clock cycles contributed by each type of instruction:
- Branch instructions:
$2.25\times {10}^8$
- Load instructions:
$1.20\times {10}^8$
- Store instructions:
$1.65\times {10}^8$
- Arithmetic instructions:
$1.30\times {10}^8$

Summing these values gives the total clock cycles:
$\text{Total Clock Cycles}=(4.5+6.0+6.6+3.9)\times {10}^8$
$\text{Total Clock Cycles}=21.0\times {10}^8$
$\text{Total Clock Cycles}=2.1\times {10}^9$ cycles.

Step 2: Relate total clock cycles to execution time
The total execution time is related to the number of clock cycles and the duration of one clock cycle by the formula:
$\text{Execution Time}=\text{Total Clock Cycles}\times \text{Clock Cycle Duration}$

Given that the execution time is $6.3$ seconds:
$6.3=2.1\times {10}^9$ × Clock Cycle Duration

Solving for the clock cycle duration:
$\text{Clock Cycle Duration}=\frac{6.3}{2.1\times {10}^9}=3.0\times {10}^{-9}$ seconds.

Since ${10}^{-9}$ seconds is equivalent to 1 nanosecond (ns), the duration of a clock cycle is:
$3\text{ns}$