Twin​ brothers, Billy and​ Bobby, can mow their​ grandparent's lawn together in 99 minutes. Billy could mow the lawn by himself in 35 minutes less time that it would take Bobby.

Answer 1

1/x + 1/y = 1/99

x=y-35

1/(y-35) + 1/y = 1/99

multiply by 99y(y-35) to eliminate fractions

99y + 99y-35(99) = y^2 -35y

solve for y= Bobby's time to mow the lawn, then subtract 35 to get Billie's time