We need to do a system of equations here.
x + y = 275 (if you travel x km by bike and y km by bus, then you travel 275 km as given in the problem)
y = x + 55 (the problem stated that they were bussed (y) the amount they biked plus 55 more km (x + 55))
The second equation is already solved for y. So, we can plug it in to the first equation.
x + (x + 55) = 275
2x + 55 = 275 [combine x terms]
2x = 220 [subtract 55 from both sides]
x = 110 [divide by 2 to isolate x and solve for it]
Now we know that x is 110, the distance they traveled by bike.
And that's what we needed to answer the problem!