pkjag:
It's simple base addition let's use base ₉:
So 9₉+9₉+9₉=30
Those who have done computer science or electrical engineering will get it, for those who don't, we operate using base₁₀, so for base₉ let's all calculations such as 4+4=8 are the same as base₁₀, but when it comes to calculations like
9+9= 20, it's just like base₂ where
1₂+1₂=10.

You see in base₉ the number 9 is the last number, so there's no 10, so when you add the last numbers together you get 20 for arithmetic convenience, how? well in base₉ there are 10 numbers: 0,1,2,3,4,5,6,7,8,9 so it'd expected that if you add the tenth number to itself it should give the twentieth number then if you add again it should give the thirtieth number.

obum88:
[15]+[13]+[7]= 30

mozeybee:
To solve this problem, you must understand algebraic operations in different number bases.
1) you must note that the normal operation will perform daily is in "base 10" ( which is an even number).
2) The sum of any three even numbers in any "even base" will always give an even number.
3) Similarly, the sum of any three odd numbers will only give an even result when operated in an "odd base"
4) Since the question did not specify which "base" the summation is operated in, one is at liberty to fix any arbitrary base for solution.
5) Since the three boxes must be filled with three odd numbers ( as provided), an "odd base" must be used.
6) The unique rule for number base operation is that " no digit in a number must be = or > the base of the number".
7) Hence, the suggestions of 9 base 9 is mathematically wrong although you will get a correct answer by 27/9= 30 (in base 9).
To solve this problem, numbers must be chosen in "base 7".
9) The basic idea of this algebraic operation is that "once the since the summation is = or > the base, the result of the summation is divided by the base and the result is the summation in that base.
10) For the solution, I pick 1, 11 and 15 (all in base 7) Note that all the digits in each number are < 7. Hence, mathematically valid.
11) Now
T U
1
+ 1 1
+ 1 5
.......................
2 7
12) Since the unit result is = to the base(7), we divide by 7 and round the whole whole answer to the Tens column, while the remainder is left in the Unit column.. Hence, 7/7 = 1 remainder 0. Hence the final result is
T U
1
+ 1 1
+ 1 5
.......................
3 0
..............,......
Q.E.S..

pkjag:
It's simple base addition let's use base ₉:
So 9₉+9₉+9₉=30
Those who have done computer science or electrical engineering will get it, for those who don't, we operate using base₁₀, so for base₉ let's all calculations such as 4+4=8 are the same as base₁₀, but when it comes to calculations like
9+9= 20, it's just like base₂ where
1₂+1₂=10.

You see in base₉ the number 9 is the last number, so there's no 10, so when you add the last numbers together you get 20 for arithmetic convenience, how? well in base₉ there are 10 numbers: 0,1,2,3,4,5,6,7,8,9 so it'd expected that if you add the tenth number to itself it should give the twentieth number then if you add again it should give the thirtieth number.

Niyoung:


Simply: [3] + [7] + [9] + [11] = 30

Emmanuelemman3:
[8] + [10] + [12] = 30

emmyrichie:


It's an aptitude test question from Apple and Microsoft.

mozeybee:
To solve this problem, you must understand algebraic operations in different number bases.
1) you must note that the normal operation will perform daily is in "base 10" ( which is an even number).
2) The sum of any three even numbers in any "even base" will always give an even number.
3) Similarly, the sum of any three odd numbers will only give an even result when operated in an "odd base"
4) Since the question did not specify which "base" the summation is operated in, one is at liberty to fix any arbitrary base for solution.
5) Since the three boxes must be filled with three odd numbers ( as provided), an "odd base" must be used.
6) The unique rule for number base operation is that " no digit in a number must be = or > the base of the number".
7) Hence, the suggestions of 9 base 9 is mathematically wrong although you will get a correct answer by 27/9= 30 (in base 9).
To solve this problem, numbers must be chosen in "base 7".
9) The basic idea of this algebraic operation is that "once the since the summation is = or > the base, the result of the summation is divided by the base and the result is the summation in that base.
10) For the solution, I pick 1, 11 and 15 (all in base 7) Note that all the digits in each number are < 7. Hence, mathematically valid.
11) Now
T U
1
+ 1 1
+ 1 5
.......................
2 7
12) Since the unit result is = to the base(7), we divide by 7 and round the whole whole answer to the Tens column, while the remainder is left in the Unit column.. Hence, 7/7 = 1 remainder 0. Hence the final result is
T U
1
+ 1 1
+ 1 5
.......................
3 0
..............,......
Q.E.S..

emmyrichie:
[ ] + [ ] + [ ] = 30

Fill the boxes using

(1, 3, 5, 7, 9, 11, 13, 15)

You can also repeat the nos.

affigurl5:
(7)+(13)+(15)=30

cassidy1996:
[5+5]+[9]+[11= 30]

Olucheye:
use another operand within the boxes,

It's as simple as,

[15] + [11] + [{9-5}] = 30

PS: The answer lies within your knowledge of how brackets work.