Daniel had 400 stamps in his stamps collection. 3/10 of them were foreign stamps. Edmund had 600 stamps and 13/20 of them were foreign stamps. After exchanging some stamps with each other, 1/4 of Danny's stamps and 3/4 of Edmund's stamps were foreign stamps. How many stamps did Danny get from Edmund?
One answer from PSLE support @Eric HK Liau
and my daughter tried her own way
Foreign (D) = 3/10 x 400 = 120, Local (D) = 280, Total (D) = 400 Foreign (E) = 13/20 x 600 = 390, Local (E) = 210, Total (E) = 600 Total foreign = 510 Total local = 490 Total stamps = 1000
Internal transfer: Total foreign and total stamps not changed.
At the end 1/4D + 3/4E = 510 (1) D + E = 1000 (2) ------------------------ D + 3E = 2040 (1) D + E = 1000 (2) ------------------------- 2E = 1040, E = 520 so D = 1000 - 520 = 480
Extra stamp Danny got from Edmund = 480 - 400 = 80
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