chetan2u wrote:
gracie wrote:
How many four digit numbers are there such that the last two digit grouping is three times the first two digit grouping?
A. 12
B. 18
C. 24
D. 30
E. 36
let number be ABCD
Let us take restrictions on the first digit from left, A - it cannot be greater than 3 as 4_*3 will result in a 3-digit number and it cannot be 0
1) let A be 1, B can be anything from 0 to 9 as even the largest 19*3=57 < 100....so 10 ways
2) let A be 2, B can be anything from 0 to 9 as even the largest 29*3=87 < 100....so 10 ways
3) let A be 3, B can be anything from 0 to 3 as 34*3=102, a 3-digit number and cannot be equal to CD....so 4 ways
total = 10+10+4=24
C
According to me correct choice would be D. 30
In addition to the 24 nos which you explained there are 6 more..
4. Let A be 4-9 and B =0; 4012, 5015, 6018, 7021, 8024, 9027.
Total 10+10+4+6 = 30