How to gain speed and avoid extra calculations in quantitative part?

Look at below five example to do those required value instead of long / lengthy calculation.
Some question need just 30 second to get the right answer. The examiner is very expert / cunning that he will ask you any thing based on general reasons and keep in mind the time is very less. So,  just focus on given MCQs instated of seeing here and there in search of help. Furthermore, there are 5 different color booklets  in GAT general / Local GRE test and you are not be able to cheat because all question are in different sequence in 5 booklets.

Note: In below example-1 if the examiner is asking to find 5/6*n  value then don't find n means avoid unnecessary calculation will save time and you will beat quantitative part easily.    

Q.1  If    5/12*n = 120  what is the value of 5/6*n = ?

 A. 6
 B. 16
 C. 61
 D. 60
 E. 66

Answer: D. 60        Sol.   5/6*n = 120/2 = 60

Q.2   If    5/12*n = 120  what is the value of n = ?

 A. 280
 B. 228
 C. 288
 D. 822
 E. 888

Answer: C = 288       Sol.   => 5n = 120 x 12 => n = 1440/ 5 = 288

Q.3   If    5/12*n = 120  what is the value of (5/12*n )^2 = ?

 A. 14000
 B. 14400
 C. 14440
 D. 14444
 E. 44444

Answer: B: 14400  Sol.  => 120^2= 14400

Q.4 If    5/12*n = 120  what is the value of (5/6*n )^2 = ?

A. 3000
 B. 3660
 C. 3600
 D. 36000
 E. 14400

Answer: C: 3600  Sol:  60^2 = 3600 

Q.5   If    5/12*n = 120  what is the value of n^2 = ?

 A. 288
 B. 82944
 C. 94482
 D. 82444
 E. 94444

Answer: B: 82944    Sol: First we find n=?  => => 5n = 120 x 12 => n = 1440/ 5 = 288

                     n= 288 and n^2 = 288^2 = 82944 

Many more possibilities are possible i.e. n^3 or 2(n)^3....etc. So, complete the requirement of questions. Wish you good luck.