Riordan had 6 times as many marbles as Bobby at first. Then each of them bought an equal number of new marbles. In the end, Bobby had 15 marbles and Riordan had 4 times as many marbles as Bobby. How many marbles did both of them buy altogether?
| |
Riordan |
Bobby |
Difference |
| Before |
6x3 =
18 u |
1x3 =
3 u |
5x3 =
15 u |
| Change |
+ ? |
+ ? |
|
| After |
4x5 =
20 u |
1x5 =
5 u |
3x5 =
15 u |
The difference in the number of marbles at first and in the end remains unchanged. Make the difference in the number of marbles at first and in the end the same. LCM of 5 and 3 is 15.
Number of marbles that Bobby had in the end = 5 u
5 u = 15
1 u = 15 ÷ 5 = 3
Number of marbles that Riordan bought
= 20 u - 18 u
= 2 u
Number of marbles that both of them bought altogether
= 2 x 2 u
= 4 u
= 4 x 3
= 12
Answer(s): 12
