Pipes A and B can fill a cistern in 10 and 90 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?
Choose the most appropriate option:
A) 13
B) 12
C) 16
D) 18
E) 19
:   1/10 + 1/90 = 1/9   9 * 2= 
<gadget id="calculator">9 * 2</gadget>
<output>18</output>
18.Answer:18
<result>D</result>
Pipes A and B can fill a cistern in 10 and 90 minutes respectively. They are opened an alternate minutes. Find how many minutes, the cistern shall be full?