Responding to a pm:
This question is quite simple when done keeping the concepts of divisibility in mind. If you are not aware of the grouping concepts of divisibility, check here first:
http://www.veritasprep.com/blog/2011/04 ... unraveled/
http://www.veritasprep.com/blog/2011/04 ... y-applied/
"A number when divided by D leaves a remainder of 7"
means when the number, say N, is divided into groups of D, you get some groups of D and 7 is leftover. Think in terms of balls. N balls divided into groups of D balls each and 7 balls leftover.
"when divided by 3D leaves a remainder of 20"
This means that when you put three-three groups of D together, you are left with 20 balls. From where did the extra 20 - 7 = 13 balls come? They must have come from one or two groups of D which were leftover. Since 13 is not a multiple of 2, we would not have had 2 groups of D balls. 13 must represent a single group of D balls. So D must be 13.
Now the question is very simple. When you divide N by 3D, which is 3*13 = 39, you are left with 20 balls. One value of N that satisfies this condition is 20. Now, when you divide 2N i.e. 40 by 3D i.e. 39, remainder will be 1.
Answer (A)![]()
Quote:
A number when divided by D leaves a remainder of 7 and when divided by 3D leaves a remainder of 20. What is the remainder left when twice the number is divided by 3D ?
a) 1
b) 13
c) 20
d) 31
e) 70
a) 1
b) 13
c) 20
d) 31
e) 70
This question is quite simple when done keeping the concepts of divisibility in mind. If you are not aware of the grouping concepts of divisibility, check here first:
http://www.veritasprep.com/blog/2011/04 ... unraveled/
http://www.veritasprep.com/blog/2011/04 ... y-applied/
"A number when divided by D leaves a remainder of 7"
means when the number, say N, is divided into groups of D, you get some groups of D and 7 is leftover. Think in terms of balls. N balls divided into groups of D balls each and 7 balls leftover.
"when divided by 3D leaves a remainder of 20"
This means that when you put three-three groups of D together, you are left with 20 balls. From where did the extra 20 - 7 = 13 balls come? They must have come from one or two groups of D which were leftover. Since 13 is not a multiple of 2, we would not have had 2 groups of D balls. 13 must represent a single group of D balls. So D must be 13.
Now the question is very simple. When you divide N by 3D, which is 3*13 = 39, you are left with 20 balls. One value of N that satisfies this condition is 20. Now, when you divide 2N i.e. 40 by 3D i.e. 39, remainder will be 1.
Answer (A)