Motor effiency

[steve.draves]

I just recently took my masters exam and missed passing it by only 3 questions.
I had a couple of questions that are still stuck in my head.

2 motors
Motor #1 115v 1/2 hp 80% eff.
Motor #2 115v 1/2 hp 90% eff.

What percent more eff. is motor #2 than Motor #1?
A. 11%
B. 24%
C. 28%
D. 60%

I chose answer A. it seemed to make the most since, but using the formula I didnt get 11%. Can anyone help me set up this question in a more useful formula?
Thanks, Steve

 

[erickench]

Motor#1 .5x746x.8=298.4
Motor#2 .5x746x.9=335.7

335.7/298.4=1.125, 1.125-1=.125 12.5%

The answer is 12.5%.

 

[jumper]

Motor#1 .5x746x.8=298.4
Motor#2 .5x746x.9=335.7

335.7/298.4=1.125, 1.125-1=.125 12.5%

The answer is 12.5%.

What is the percentage value of the fraction 1/9?

 

[jim dungar]

The motor #2 is 12.5% more efficient than the motor #1.
The motor #1 is 11.1% less efficient then the motor #2.

Your are looking at the % difference, not the difference in %.

The formulas are:
percent difference of low value = (H-L)/L = (90-80)/80 = .125= 12.5%
percent difference of high value = (H-L)/HV = (90-80)/90 = .111= 11.1%

 

[jumper]

The motor #2 is 12.5% more efficient than the motor #1.
The motor #1 is 11.1% less efficient then the motor #2.

Your are looking at the % difference, not the difference in %.

The formulas are:
percent difference of low value = (H-L)/L = (90-80)/80 = .125= 12.5%
percent difference of high value = (H-L)/HV = (90-80)/90 = .111= 11.1%

1/8 vs 1/9. I get it thanks.