The aim is to move all the frogs across to the other side of the river.. Move the frogs forward or backward so that the total distance travelled equals the target fraction.
In the first example on the left two frogs have been moved forward. An outline is left behind to show where a frog started.
The total move is 1/4 + 1/12 = 3/12 + 1/12 = 4/12 = 1/3
If the target fraction is 7/8 then one frog could be moved forward 1/2 and another forward 3/8.
If the target is 3/8 then one frog could go forward 1/2, another could be moved back 1/8. ie 1/2 - 1/8 = 4/8 - 1/8 = 3/8.
In the second example on the left one frog has been moved forward and the other moved backward. An outline is left behind to show where the frogs started.
The total move is 2/9 + 1/12 = 8/36 + 3/36 = 11/36