Jack usually mows his lawn in 3 hours. Marilyn can mow the same yard in 7 hours. Home much time would it take for them to mow the lawn together?

Question

Simplify the answer 

Answer 1

time together/time alone + time together/time alone = 1

Let t = the time it would take for them to mow the yard together

t/3 + t/7 = 1       Multiply both sides by 21.

7t + 3t = 21

10t = 21

t = 2.1 hours

 

Answer 2

(x)(1/3) + (x)(1/7) = 1

 

(7x/21) + (3x/21) = 1

 

10x/21 = 1

 

10x = 21

 

x = 21/10 = 2.1 hours

 

ChecK:

 

2.1 hr(1 job/3hr) + 2.1 hr(1 job/7hr) = 1 hr

 

2.1/3 + 2.1/7 = 1 job

 

.7 + .3 = 1