Irene has a bag containing some silver and black stamps. If she adds in 17 silver stamps, 20% of the stamps in the bag are black. If she adds in another 55 black stamps, 30% of the stamps in the bag are black. How many silver stamps are in the bag?
| |
Silver stamps |
Black stamps |
| Before |
28 u - 17 |
7 u |
| Change 1 |
+ 17 |
|
| After 1 |
4x7 =
28 u |
1x7 =
7 u |
| Change 2 |
|
+ 55 |
| After 2 |
7x4 =
28 u |
3x4 =
12 u |
20% =
20100 =
1530% =
30100 =
310The number of silver stamps remains unchanged at the second change when she adds in another 55 black stamps. Make the number of silver stamps the same at the second change. LCM of 4 and 7 is 28.
Number of black stamps added
= 12 u - 7 u
= 5 u
5 u = 55
1 u = 55 ÷ 5 = 11
Number of silver stamps at first
= 28 u - 17
= 28 x 11 - 17
= 308 - 17
= 291
Answer(s): 291
