Twin brothers, Billy and Bobby, kid mother grandparents lawn together for 60 minutes. Billy could mow the lawn by himself in 20 minutes less time then it would take Bobby. How long will it take Bobby to mow the lawn by himself

Bobby around 131 minutes and Billy around 111 minutes

Step-by-step explanation:

To solve the problem it is important to raise equations regarding what happens.

They tell us that Billy (Bi) and Bobby (Bo) can mow the lawn in 60 minutes. That is to say that what they prune in a minute is giving as follows:

1 / Bo + 1 / Bi = 1/60 (1)

They say Billy could mow the lawn only in 20 minutes less than it would take Bobby, therefore

1 / Bi = 1 / (Bo-20) (2)

Replacing (2) in (1) we have:

1 / Bo + 1 / (Bo-20) = 1/60

Resolving

(Bo - 20 + B0) / (Bo * (Bo-20) = 1/60

120 * Bo - 1200 = Bo ^ 2 - 20Bo

Rearranging:

Bo ^ 2 - 140Bo - 1200 = 0

Now applying the general equation

Bo = 130.82 or Bo = 9.17, this last value cannot be because Billy took 20 minutes less and neither can he prune faster than the two together, therefore Bobby only takes around 131 minutes and Billy around 111 minutes

Checking with equation 1:

1/131 + 1/111 = ~ 1/60