Question 70
The mean of all 4-digit even natural numbers of the form 'aabb',where $$a>0$$, is
Solution
The four digit even numbers will be of form:
1100, 1122, 1144 ... 1188, 2200, 2222, 2244 ... 9900, 9922, 9944, 9966, 9988
Their sum 'S' will be (1100+1100+22+1100+44+1100+66+1100+88)+(2200+2200+22+2200+44+...)....+(9900+9900+22+9900+44+9900+66+9900+88)
=> S=1100*5+(22+44+66+88)+2200*5+(22+44+66+88)....+9900*5+(22+44+66+88)
=> S=5*1100(1+2+3+...9)+9(22+44+66+88)
=>S=5*1100*9*10/2 + 9*11*20
Total number of numbers are 9*5=45
.'. Mean will be S/45 = 5*1100+44=5544.
Option D
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