Question 472741
The first thing that you should understand here is that if the planes start at the same time then at the point at which they meet, the time will be the same t=3hr. In addition, the total distance travelled by the two planes  will be equal to the sum of their individual distances travelled  x+y=2700


speed of plane  A  Va=Vb+100kph  the plane travelled 2700-y distance in t=3 hr
speed of plane B  is Vb the plane travelled 2700-x distance in t=3 hr

using formula of the speed {{{v=d/t}}}  
for the plane A we write

{{{Va= x/t}}}
{{{Va=(2700-y)/3}}}
{{{Vb+100=(2700-y)/3}}}
{{{Vb=(2700-y)/3-100}}}

for the plane B

{{{Vb=y/t}}}
{{{Vb=y/3}}}

Now compare two equations Vb=Vb
{{{(2700-y)/3-100=y/3}}}  multiply both sides by 3
{{{2700-y-300=y}}}
2y=2400
y=1200

then x= 1500


having the distance x and y the planes travelled in t=3hr you can find  speed of each plane

Va= x/t= 1500/3=500 kph
Vb=y/t=1200/3=400kph