Physics probs need help!!!?

Question

Ok these 2 i had the most difficult with.

1- Suppose you have to hold a box of mass 10 kg against a wall so that the box does not slip. The coefficient of friction between the wall and the box is 0.70. What must be the minimal force you have to apply?

2- 3 masses m1 = 10 kg, m2 = 15 kg and m3 = 20 kg, are pulled with a force of 166N across a table. A coefficient of friction between the table and boxes is 0.12.
(theres suppose to be a pic but i couldnt get it on here)

a- Calculate the acceleration of the masses
b- Calculate the tension of T2
c- Calculate the tension T3

Answer 1

1 Weigh of the box W must equal to that of the friction f

W=f

W=mg
W=uN
N=W/n
N= mg/u= 10 x 9.81/0.7=130N

2.

a) F=ma

Ft=F-f
a=Ft/(mt)
a=[ F - (f1 + f2 + f3)]/(m1+ m2 + m3)
a=[F- gu(m1 + m2 +m3)]/(m1+ m2 + m3)
a= F/(m1+ m2 + m3) - gu
a= 166/(45) - 9.81 (0.12)=2.51m/s^2

b) T2=m2a2= 37.7 N

c) T3= m3a3=50.2 N