Question 938539
Failure rate for each instrument, p = 0.0047
It is a requirement that the two instruments are independent, which means that the probability that both will fail is governed by the multiplication rule, i.e.
Probability of both (independent) instruments fail 
= p*p = {{{0.0047^2}}} 
= {{{2.209 10^(-5)}}}
Probability that at least one instrument will be functional
= 1-p*p
= 0.99997791
  
Rounding to three significant figures will be misleading because the correct answer is very close to 1.  By rounding to three significant figures the result is 1.000, which has a very different interpretation from that of the correct answer.
  
I do not know the required safety standard.  Generally if p is small enough, doubling with an independent instrument will reduce the probability of both instruments failing to very small value.