Мастер выполняет весь объем работы за 4 ч, а его ученик за 12, за какое время выполнят эту работу мастер и ученик, работая вместе?

Помогите, пожалуйста!

II. Вставьте глаголы в нужной форме:

1. I don't know how the cat __________________ get through the window, but it did. (manage)
2. I'll post your letter, but I don't think the __________________ read the address. (to be able to)
3. Who the piano upstairs?' 'My sister. She's got a concert tomorrow.' (play)
4. She to have a problem now. (appear)
5. Not many passenger planes __________________ faster than sound. (fly)
6. I __________________ Rob recently. (not see)
7. I __________________ to be a doctor when I was fifteen. (want)
8. He very hard this week. (work)
9. It __________________ since I got up. (snow)
10. All the lights today.( switch on )
11. He to come soon.(to expect)
12. I don't want her __________________ everybody. (tell)
13. If she __________________ late again, she'll lose her job. (come)
14. 1 shouldn't worry if I you. (be)
15. Pam rang to say that John __________________ an accident. (have)
16. The winner a prize at the competition yesterday. (give)
17. He found out that the Caf? Royal into a casino. (turn)
18. A full explanation __________________ to us at the last meeting. (promise)
19. When I entered they__________________ about this problem. (talk)
20. The professor said that Shakespeare__________________ French. (not speak)