Skag55 wrote:

I remember having seen a formula for rates

\frac{A*B}{A+B} but I can't remember what it is for (if anything at all). Can someone clarify this for me?

Thanks

THEORYThere are several important things you should know to solve work problems:

1. Time, rate and job in work problems are in the same relationship as time, speed (rate) and distance in rate problems.time*speed=distance <-->

time*rate=job \ done. For example when we are told that a man can do a certain job in 3 hours we can write:

3*rate=1 -->

rate=\frac{1}{3} job/hour. Or when we are told that 2 printers need 5 hours to complete a certain job then

5*(2*rate)=1 --> so rate of 1 printer is

rate=\frac{1}{10} job/hour. Another example: if we are told that 2 printers need 3 hours to print 12 pages then

3*(2*rate)=12 --> so rate of 1 printer is

rate=2 pages per hour;

So, time to complete one job = reciprocal of rate. For example if 6 hours (time) are needed to complete one job --> 1/6 of the job will be done in 1 hour (rate).

2. We can sum the rates.If we are told that A can complete one job in 2 hours and B can complete the same job in 3 hours, then A's rate is

rate_a=\frac{job}{time}=\frac{1}{2} job/hour and B's rate is

rate_b=\frac{job}{time}=\frac{1}{3} job/hour. Combined rate of A and B working simultaneously would be

rate_{a+b}=rate_a+rate_b=\frac{1}{2}+\frac{1}{3}=\frac{5}{6} job/hour, which means that they will complete

\frac{5}{6} job in one hour working together.

3. For multiple entities: \frac{1}{t_1}+\frac{1}{t_2}+\frac{1}{t_3}+...+\frac{1}{t_n}=\frac{1}{T}, where T is time needed for these entities to complete a given job working simultaneously.For example if:

Time needed for A to complete the job is A hours;

Time needed for B to complete the job is B hours;

Time needed for C to complete the job is C hours;

...

Time needed for N to complete the job is N hours;

Then:

\frac{1}{A}+\frac{1}{B}+\frac{1}{C}+...+\frac{1}{N}=\frac{1}{T}, where T is the time needed for A, B, C, ..., and N to complete the job working simultaneously.

For two and three entities (workers, pumps, ...):

General formula for calculating the time needed for two workers A and B working simultaneously to complete one job:Given that

t_1 and

t_2 are the respective individual times needed for

A and

B workers (pumps, ...) to complete the job, then time needed for

A and

B working simultaneously to complete the job equals to

T_{(A&B)}=\frac{t_1*t_2}{t_1+t_2} hours, which is reciprocal of the sum of their respective rates (

\frac{1}{t_1}+\frac{1}{t_2}=\frac{1}{T}).

General formula for calculating the time needed for three A, B and C workers working simultaneously to complete one job:T_{(A&B&C)}=\frac{t_1*t_2*t_3}{t_1*t_2+t_1*t_3+t_2*t_3} hours.

Some work problems with solutions:

time-n-work-problem-82718.html?hilit=reciprocal%20ratefacing-problem-with-this-question-91187.html?highlight=rate+reciprocalwhat-am-i-doing-wrong-to-bunuel-91124.html?highlight=rate+reciprocalgmat-prep-ps-93365.html?hilit=reciprocal%20ratequestions-from-gmat-prep-practice-exam-please-help-93632.html?hilit=reciprocal%20ratea-good-one-98479.html?hilit=ratesolution-required-100221.html?hilit=work%20rate%20donework-problem-98599.html?hilit=work%20rate%20donehours-to-type-pages-102407.html?hilit=answer%20choices%20or%20solve%20quadratic%20equation.%20RHope it helps.

_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:

PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.

What are GMAT Club Tests?

25 extra-hard Quant Tests

GMAT Club Premium Membership - big benefits and savings