Let 'x' be the velocity of one plane

'y' be the velocity of the other

knowing that the formula for velocity is defined by the equation :

where v is the velocity

d is the distance traveled

t is the time

modifying the equation you'll have:

since you are given the distance as 1,140mi and the time traveled as 1.5hours

and that the other plane is traveling 10mph faster than the other then you'll have:

xt + yt = 1,140mi -----equation 1

x = y + 10mph -----equation 2

substituting equation 2 to 1 and having the time as 1.5hrs,

xt +yt = 1,140

(y+10)(1.5) + y(1.5) = 1,140

1.5y +15 + 1.5y = 1,140

3y = 1,140 - 15

3y = 1125

__y = 375mph__substitute to equation 2

x = y + 10

x = 375 + 10

__x = 385mph__