Question

A company plans to purchase a machine who uptime needs to be atleast 95%. They have shortlisted two models of the machine with the following operational characteristics:

Machine	MTBF(hr)	MTTR(hr)
Model M	60	4
Model N	48	2

The company should buy

1. Only Model M
2. Only Model N
3. Either Model M or N
4. Neither Model M nor N

Answer 1

Correct Answer - Option 2 : Only Model N

The operational characteristics of two models of the machine

Machine	MTBF(hr)	MTTR(hr)
Model M	60	4
Model N	48	2

 

Uptime needs to be atleast 95%. i.e. uptime ≥ 95%

Now, uptime of a system is given by,

\(Uptime = \frac{{MTBF}}{{MTBF + MTTR}}\)

For model M:

\(Uptime = \frac{{MTBF}}{{MTBF + MTTR}} = \frac{{60}}{{60 + 4}} = \frac{{60}}{{64}} = 0.9375\)

= 93.75% < 95%

For model N:

\(Uptime = \frac{{MTBF}}{{MTBF + MTTR}} = \frac{{48}}{{48 + 2}} = \frac{{48}}{{50}} = 0.96\)

= 96% > 95%

∴ The company should buy only model N as the uptime is > 95% whereas is case of model M, the uptime < 95%