math
posted by rebeca .
alter working 4 hours, frank has made 21 machine parts.at the same rate how many parts can he make in 7 hours? 4(21)/7,7(21),7(21)/4,7(4)/21,7(4)(21)?

Please post your answer and we'll be glad to check it.

i choose a)4(21)/7

Look at dimensional analysis.
21 parts/4 hrs x ?hrs = ?parts. 
How can a) be the right answer? 4(21)/7 = 12. The problem has stated that he made 21 parts in 4 hours. How can he make fewer parts in more hours?
Please try again.
Hint: you could set up a proportion:
4 is to 21 as 7 is to x 
d)7(4)/21

7(4)/21.
Do the math. Does the answer make sense?