14 days ago, Xuan had a total of 250 black and grey stamps. 30% of her stamps were grey stamps. Today, her grandfather gave her some more stamps. As a result, her black stamps increased by 40%. Now, 50% of her stamps are grey stamps. How many grey stamps did her grandfather give her?
| |
Grey stamps |
Black stamps |
Total
stamps |
| Before |
3 u |
7 u |
250 |
| Change |
+ 6.8 u |
+ 2.8 u |
|
| After |
9.8 u |
9.8 u |
|
Comparing the
black stamps and
grey stamps in the end |
1 |
1 |
|
30% =
30100 =
310 Grey stamps : Black stamps = 3 : 7
Increase in the number of black stamps
= 40% x 7 u
=
40100 x 7 u
= 2.8 u
50% =
50100 =
12 In the end
Grey stamps : Black stamps = 1 : 1
Number of stamps at first
= 7 u + 3 u
= 10 u
10 u = 250
1 u = 250 ÷ 10 = 25
Number of grey stamps that Xuan received from her grandfather
= 9.8 u - 3 u
= 6.8 u
= 6.8 x 25
= 170
Answer(s): 170
