Question 442497
800/x=800/(x+100)+4
800(x+100)=800x+4x2+400x
800x+80000=4x2+1200x
0=4x2+400x-80000
0=x2+100x-20000
0=(x+200)(x-100)
x=100 or -200.   Throwing out the negative answer, we get the slow plane flying at 100mph, the fast plane moving at 200mph..