Two children A and B are playing a game. A can draw a picture in 30 minutes and B can
erase it in 40 minutes. If A starts drawing, and if the drawing sheet is passed on to these two
alternately for every one minute, then the time (in minutes) required to complete a picture
for the first time is​

240 min

240 minutes. A draws 1/30 in a minute and b erases 1/40 in a minute. The net work done in 2 mins is 1/30-1/40 i.e. 1/120. For the total painting to be finished 120 iterations of A drawing and B erasing are required. Since one iteration was 2 mins(A drawing for 1 min and B erasing for 1 min) the total time required is 120 * 2min = 240min

240 mins ?