1/4×2.4=0.6=60%

Answer; The cooperation between the two was completed in 2.4 days; A has finished 60% of the work.

(2)( 1+0.05)×( 1-5%)=0.998=99.8%

A: It is estimated that this year's tax is 99.8% of last year's.

(3) Unpack the white chalk X box.

4/5 X×( 1-25%)=X-30

X=75

75×4/5=60 (box)

The school bought 60 boxes of colored chalk.