Hey there!!
The number of mountain bikes - x and trail bikes - y
Into an equation
40x + 50y = 1750 ( this is for materials ) ------------------ ( 1 )
80x + 50y = 2750 ( this is for labor ) --------------------- ( 2 )
Subtracting the first equation from the second
40x = 1000
x = 1000 / 40
x = 25
x = mountain bikes = 25 bikes
The required answer = 25
Hope helps!