1, How we can construct an angle of 60^o.
Step1 : Draw a line segment AB
step 2 : With A as centre  and  any radius draw an arc cutting AB at C. 
Step 3: With C as centre and the same radius as before cutoff the arc at D
Step 4 : Join AD
step 5 : Then  BAD = 60^o 
              

2, How we can construct an angle of 120o?
Step: 1 Draw a line segment AB
Step 2 : With A as centre and any radius draw an arc cutting AB at C. 
Step: 3: With C as centre and the same radius as before cut off the arc first at D and  then finally at E by taking D as centre. 
Step : 4 Join AE
Step: 5 Then  BAE = 120^o.
           

3, How can we construct an angle of 30^o ? 
     First construct an angle of 60o as explained before and then bisect this angle.
           

4, Explain the construction of 90^o angle. 
Step 1 : Draw a line segment  AB
Step 2 : With A  as centre and any radius draw an arc 1 cutting AB at C
Step 3 : with C as centre  and the same radius as before draw an arc z cutting the previous arc at D.
step 4 : With D as centre and the same radius draw an arc 3 cutting the arc 1 drawn to step 2 at E.
Step 5 : Now with D and E as centres and radius more than ¹/₂ DE, draw arac s.  4 and 5 cutting each other at F. 
step 6 : Jopin at A F.
Step 7 : Then  BAF = 90^o 
   construction
   

5, Explain the construction of angle of 45^o ?
Step 1 : First draw an angle of 90^o
step 2 : Bisect this angle by AH
Step 3 :  Then  BAH = 45^o