If s+a=150. and 7.50s+150a=1612.50. how do i solve this im learning about substitution

Solve the first equation for any variable you want:

s = 150 - a

Now that you know what the value of just the variable s is equal to, you can put (150-a) in for s in the second equation.

7.5(150-a) + 150a = 1612.5

Distribute the 7.5...

1125 - 7.5a + 150a = 1612.5

Combine like terms and subtract 1125 from both sides to isolate the variable a:

142.5a = 487.5

Divide both sides by 142.5 to solve for a:

a = 3.42 (I rounded my answer)

Now that you know what a is equal to, you can put it back into your equation (s = 150 - a) to solve for what S is.

s = 150 - 3.42

s = 146.58