document.write( "Question 503580: A 90,000 gallon water tank can be filled in two hours by opening valve A alone and in two and a half hours by opening valve B alone. It can be emptied in three hours by opening valve C alone. How long will it take to fill the tank under each of the following conditions? If an answer is not whole hours, include hours, minutes and seconds in your answer.\r \n" );
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document.write( " a. Valve B is kept closed. Valves A and C are opened \n" );
document.write( " b. Valve C is kept closed. Valves A and B are opened \n" );
document.write( " c. Valve A is kept closed. Valves B and C are opened \n" );
document.write( " d. valve C is kept closed. Valve A is opened. Twenty minutes later valve B is also opened \n" );
document.write( " e. Valves A and C are opened. Twenty minutes later valve B is also opened \n" );
document.write( " f. Valve A is opened. Twenty minutes later valves B and C are also opened \n" );
document.write( " g. Valve C is kept closed. Valve A is opened. Valve B is also opened twenty minutes later and is then closed after another ten minutes \n" );
document.write( " h. Valve C is kept closed. Valve A is opened. Twenty minutes later valve B is also opened. Ten minutes after that, Valve A is closed \n" );
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