2.7

3.4:9

4. Assuming that X kilograms of the first sugar is used, then:

(8.4X+7. 12 * 5)/(X+5)= 7.6x = 3。 I used 3 kilograms of sugar for the first time.

5. The original number of new books is 2 10*5/7= 150.

If you buy x more copies, then (150+x)/(210+x) = 4/5, and you get X=90.

So there are 150+90=240 new books.