a cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. if both the taps are opened simultaneously then after how much time will the cistern get filled?	Pick
A) 6 hr B) 5.6 hr C) 9.5 hr D) 7.2 hr E) 4 hr
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 / 9</gadget>
<output>1/9 = around 0.111111</output>

<gadget id="calculator">(1/4) - (1/9)</gadget>
<output>5/36 = around 0.138889</output>

<gadget id="calculator">1 / (5/36)</gadget>
<output>36/5 = around 7.2</output>

<result>D</result>
a cistern can be filled by a tap in 4 hours while it can be emptied by another tap in 9 hours. if both the taps are opened simultaneously then after how much time will the cistern get filled?
divide(const_1, subtract(divide(const_1, 4), divide(const_1, 9)))
"net part filled in 1 hour 1 / 4 - 1 / 9 = 5 / 36 the cistern will be filled in 36 / 5 hr = 7.2 hr answer is d"