Question 509028
let speed of plane = xmph

time against = time with wind

time against = 2100/(x-60)
time with wind = 2400/(x+70)

2100/(x-60)= 2400/(x+70)

2100(x+70)=2400(x-60)

2100x+147000=2400x-144000

2400x-2100x=147000+144000
300x=294000
/300
x=960 mph