1 * 1 / 2 + 2 * 1 / 3 + 3 * 1 / 4 +9 * 1 / 10 is calculated by a simple method

1 * 1 / 2 + 2 * 1 / 3 + 3 * 1 / 4 +9 * 1 / 10 is calculated by a simple method

=(1-1/2)+(1/2-1/3)+(1/3-1/4)+.-1/10=1-1/10=9/10

99 times 0.24 plus 76 times 0.99 is calculated by a simple method

99×0.24 + 76 ×0.99
= 99×0.24 + 0.76×99
= 99 ×（0.24 + 0.76）
= 99× 1
= 99

How many zeros are there at the end of the product of 1 × 2 × 3 × 4. × 79 × 80?

If there are several factors 5, there will be several zeros at the end, because only 2 * 5 can provide 0 at the end, and obviously factor 2 is much more than 5
5. The 16 numbers of 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75 and 80 can only provide one factor except 25, 50 and 75. There are 19 factors 5 in total, so there are 19 zeros at the end of the product

What is 1-2 + 3-4 +. + 79-80?