The answers are: 1. Sara has been making cakes for three hours. 2. Sara has made 100 cakes. 3. They have been repairing cars since 1999. 4. They have repaired 3000 cars. When the action is happening over a period of time in the past and continues until now, you use the past tense of "to be" (was, were) and a present participle (the verb form ending in -ing). If the action is completely finished, you use the simple past tense or the present tense of the verb "to have" with the past participle (the verb form ending in -ed, usually).

Grimm is %100 right !

1- has been making because for three hours . 2-has made because she's finished cooking and we sea 100 cakes. 3-have been repairing because since 1999. 4-have repaired because 3000 cares. That is my answer . Thanks