Use the formula
to obtain
distance = rate *time,
so, time = distance /rate -----**
time in going 4200km away = 4200/x hrs.
time in going back = 4200 /(x +100) hrs.
4200/x +4200/(x+100) = 13
Clear the fractions, multiply both sides by x(x+100),
4200(x+100) +4200(x) = 13(x)(x+100)
4200x +420,000 +4200x = 13x^2 +1300x
13x^2 -7100x -420,000 = 0
Using the Quadratic formula,
x = {7100 +,-sqrt[7100^2 -4(13)(-420,000))]} /2(13)
x = 600 km/hr, or -53.85 km/hr
Therefore, the plane was going originally at 600 km/hr. -----answer.