Calculating Mass of Little Sister Using Newton's Second Law

[Propaganda777]

You are pulling your little sister on her sled acroos an icy(frictionless) surface. When you exert a constant horizontal force of 110 N, the sled has an acceleration of 2.5 m/s^2. If the sled has a mass of 7 kg, what is the mass of your little sister?

Heres how I attempted it:
F=110N
A=2.5 m/s^2
M1= 7kg
M2= ?

F=(M1+M2) A
M2= 41.2 kg


The answer is 37 kg, but I can't figure out how to arrive at this mass. Can anyone help me?

 

[Doc Al]

F=(M1+M2) A

Since you know F and A, you can solve for M1 + M2. Then just subtract out M1.

 

[Propaganda777]

I did, but it comes out to a wrong answer:

F= (m1+m2)a

m2=(F-m1)/a

m2=41.2kg

The answer should be 37 kg, right?

 

[Doc Al]

I did, but it comes out to a wrong answer:

F= (m1+m2)a

m2=(F-m1)/a

That last step doesn't follow. Do this:
F = (m1 + m2)a
F/a = m1 + m2

What's that equal?

 

[Propaganda777]

hey it works, thanks you:)