If the speed of an aeroplane is reduced by 40 km/hr, it takes 20 minutes more to cover 1200 km. Find the speed of the aeroplane.
If the speed of an aeroplane is reduced by 40 km/hr, it takes 20 minutes more to cover 1200 km. Find the speed of the aeroplane.

How about we think about the first speed of the plane to be x km/hr.

Presently, the time taken to cover a distance of

    \[1200\text{ }km\text{ }=\text{ }1200/x\text{ }hrs\]

[Since, Time = Distance/Speed]

Leave the new speed of the plane alone

    \[\left( x\text{ }\text{ }40 \right)\text{ }km/hr.\]

In this way, the new time taken to cover a distance of

    \[1200\text{ }km\text{ }=\text{ }1200/\left( x\text{ }\text{ }40 \right)\text{ }hrs\]

As per the inquiry, we have

Concise Selina Solutions Class 10 Maths Chapter 6 ex. 6(C) - 3

    \[x\left( x\text{ }\text{ }40 \right)\text{ }=\text{ }48000\text{ }x\text{ }3\]

    \[x2\text{ }\text{ }40x\text{ }\text{ }144000\text{ }=\text{ }0\]

    \[x2\text{ }\text{ }400x\text{ }+\text{ }360x\text{ }\text{ }144000\text{ }=\text{ }0\]

    \[x\left( x\text{ }\text{ }400 \right)\text{ }+\text{ }360\left( x\text{ }\text{ }400 \right)\text{ }=\text{ }0\]

    \[\left( x\text{ }\text{ }400 \right)\text{ }\left( x\text{ }+\text{ }360 \right)\text{ }=\text{ }0\]

As, speed can’t be negative. So we just take,

    \[x\text{ }=\text{ }400.\]

In this way, the first speed of the plane is

    \[400\text{ }km/hr.\]