1, a type x is set every minute, so the type x-60 is set every minute.

So 40x=60(x-60), so x= 180.

So * * * has 40* 180=7200 words.

It takes 7200/(180+120) = 24 minutes for two people to play together.

2. Your teacher's efficiency is not written. I use an a here. You can replace it then.

Suppose * * * has x parts, then the master has processed 4/9x and the apprentice has processed 5/9x.

So (5/9x-240)/25 = (4/9x)/a.

Just work out X, and the processing number of master and apprentice is very simple.

3、 1/( 1*3)+ 1/(3*5)+ 1/(5*7)+……+ 1/(2 1*23)

= 1/2( 1- 1/3)+ 1/2( 1/3- 1/5)+ 1/2( 1/5- 1/7)+……+ 1/2( 1/2 1- 1/23)

= 1/2( 1- 1/3+ 1/3- 1/5+ 1/5- 1/7+ 1/7+……+ 1/2 1- 1/23)

= 1/2( 1- 1/23)= 1 1/23