Question
A man can go on a motor boat with a speed of 15 km/hour
in still water, he goes to a certain distance upstream and come back to the starting point in a river which flows at a speed of 5km/hour. Find the average speed of motor boat for total journey?Solution
Let, the speed of boat be u km/hour and speed of current be v km/hour Average speed = 2xy/(x+y) (here, x is speed of downstream and y is speed of upstream) = (2 (u+v)(u-v))/((u+v+u-v)) = (u2-v2)/u = (225-25)/15 = 40/3 km/hour IXAMBEE Method :- 
Average speed = (Total distance covered)/(total time taken) = (2×60)/9 =40/3 km/hour
16, 27, 44, 63, 98, 139
210, 140, 105, 84, 70, 63, 52.5Â
0 7 26 63 126 215
...32, 46, 88, 158, 256, 380
Find the wrong number in the given number series.
32, 48, 84, 146, 248, 392
100, 75, 59, 55, 46, 45
21Â Â Â Â Â Â Â Â Â Â 32Â Â Â Â Â Â Â Â Â Â 54Â Â Â Â Â Â Â Â Â Â 86Â Â Â Â Â Â Â Â Â Â 131Â Â Â Â Â Â Â Â 186
...131, 212, 261, 286, 295, 299
Find the wrong number in the given number series.
3, 7, 22, 89, 445, 26771, 2, 8, 48, 386, 3840
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