1. When weighing a 6 kg object, the spring elongation is13.5-12,5 =1cm. According to this proposal, the following proportional formulas can be listed:

6: 1=5:x

X=5/6=0.83 cm,

The original length of the spring is 12.5 cm, so when using a 5 kg object, the length of the spring is12.5+0.83 =13.33 cm.

2. The original proportion of people who reached the standard was 3/(3+5)=3/8.

9/20-3/8=3/40 is the proportion after an increase of 60 people.

60/(3/40)=800 people

3.0.9 1/(4/5)- 1.25*0.7 1

=0.9 1*(5/4)-(5/4)*0.7 1

=(5/4)*(0.9 1-0.7 1)

=(5/4)*0.2

=(5/4)*( 1/5)

= 1/4

=0.25