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This essential trains for: SATI, GMAT, AMC8, Math Kangaroo 78, Math Kangaroo 56.
For age problems, observe the following 2 rules:
Time flows for everyone.  Jed is three times the age of Ned. Five years from now, Jed will be twice older than Ned. How old is Ned now? 
J is the age of Jed now and N is the age of Ned now. Note how the 5 years were added to both J and N. 

An age difference does not change in time.  Ted is 45 and Zed is 17. How many years ago was Ted 5 times older than Zed? 
T denotes the age of Ted now. Z denotes the age of Zed now. Some years ago Zed's age was X and Ted's age was 5X, but the difference was the same: 28 years. At the time, Zed was 7  it happened 10 years ago. 
For coin problems, remember that the number of coins is different from the monetary value of the coins.
Example of coin problem:
In a piggy bank there are q quarters, d dimes and n nickels. There are twice as many nickels as quarters and half as many quarters as dimes. How many quarters are there if the total amount in the piggy bank is $6.50?
The most important thing is to keep in mind that q, d, and n are numbers of coins, whereas their monetary values are 0.25, 0.1, and 0.05, respectively.
Now compute the monetary value of the coins (in cents):
and set it to equal 650 cents:
There are 10 quarters in the piggy bank.
Examples of time problems:
There are 480 min in 8 hours. 9 periods of 53 minutes total 477 minutes. There are 9 3minute breaks totaling 27 minutes of break time. 

1 min = 60 s  The maximum duration of a solar eclipse has been calculated to be of 7 minutes and 30 seconds. How much percent of the longest possible duration was the duration of the 2009 eclipse? 
Convert the time to seconds in both cases. The longest possible eclipse lasts 450 seconds. The 2009 eclipse lasted 396 seconds. Denote the unknown percentage with x. Solve for x to get 88%. 

1hr = 3600 s  Hearsay has it that Bill Gates' fortune increases by $150 every second. How much money does he make by sitting around doing nothing for 8 hours a day?  There is nothing to explain, really. 