25% are dated before 1980, therefore: 1200(total):100 = 12 (1%) 12(1%)x25 = 300 (25%)
35% are dated from 1980 to 2000, therefore: 12(1%)x35 = 420 (35%)
25% are dated before 1980, 35% are dated from 1980 to 2000 and the rest are dated after 2000. 25% + 35% = 60%. 300(25%) + 420(35%) = 720. 60% = 720. 100%-60% = 40%. 40% is the amount of pennies that are dated after 2000.
The total (100%) of the pennies is 1200. We also know that 60% of the total (1200) is 720.
1200(100%) - 720(60%) = 480(40%).
Therefore, the amount of pennies dated after 2000 in Russell’s Collection is 480.