Answer:
The plane travelled for [tex]\text{$2$ hours}[/tex] at an average of speed [tex]120\; \rm mph[/tex] and [tex]\text{$3$ hours}[/tex] at an average speed of [tex]130\; \rm mph[/tex].
Step-by-step explanation:
Let [tex]x[/tex] denote the number of hours that the plane travelled at an average speed of [tex]120\; \rm mph[/tex].
Given that the trip is [tex]5\; \text{hours}[/tex] long in total, the plane would have travelled at an average speed of [tex]130\; \rm mph[/tex] for [tex](5 - x)\; \text{hours}[/tex].
The plane would have travelled [tex]120\, x[/tex] miles after [tex]x\; \text{hours}[/tex] at an average speed of [tex]120\; \rm mph[/tex]. Likewise, the plane would have travelled [tex]130\, (5 - x)\; \text{miles}[/tex] after [tex](5 - x)\; \text{hours}[/tex] at an average of [tex]130\; \text{mph}[/tex].
The plane has travelled [tex]630\; \text{miles}[/tex] in total. In other words:
[tex]120\, x + 130\, (5 - x) = 630[/tex].
Solve this equation for [tex]x[/tex]: [tex]x = 2[/tex].
In other words, the plane has travelled for [tex]\text{$2$ hours}[/tex] at an average of speed [tex]120\; \text{mph}[/tex]. It would have travelled for [tex](5 - x)\; \text{hours} = (5 - 2)\; \text{hours} = 3 \; \text{hours}[/tex] for the other part of the trip (at an average speed of [tex]130\; \text{mph}[/tex].)
