Difference in time of caesium clocks = 0.02 s

Time required for this difference = 100 years

= 100 × 365 × 24 × 60 × 60 = 3.15 × 10^{9} s

In 3.15 × 10^{9} s, the caesium clock shows a time difference of 0.02 s

In 1s, the clock will show a time difference of (0.02/3.15 x 10^{9})s

Hence, the accuracy of a standard caesium clock in measuring a time interval of 1 s is

3.15 x 10^{9}/0.02 = 157.5 x 10^{9} s ≈ 1.5 x 10^{11} s