If x+y=375 and 1.25x+0.75y = 393.75 then what is y

Y = 150

Start by using substitution.

Change x+y=375 to x=375-y

Substitute this into 1.25x+0.75y=393.75

1.25 (375-y) + 0.75y=393.75

Distribute the 1.25

468.75-1.25y+0.75=393.75

Add like terms (y)

468.75-0.5y=393.75

Subtract 468.75 to the right

-0.5y=-75

Divide by - 0.5

y=150