mathematics challenge

i got something nice this afternoon that i want to share with all of you...(courtesy of my ex-students)

If you are an Engineer, you should be able to solve it within 3 minutes or less (30 seconds);
If you are an Architects, perhaps you will need 3 hours;
If you are a Doctor, perhaps you will need 6 hours;
If you are an Accountant, maybe 3 months;
If you are a Lawyer, maybe you can never find the answer....

nasib baik lar i'm not a lawyer (^o*) dats why i ONLY need less than 10minutes to solve this...

so, kengkawan....
SILA LA TRY yer KALO BERANI (^o^)...hohoho

|---+----+----+------+-----|

| 2 | 3 | 4 | 15 | 12 |

|---+----+----+------+------|

| 3 | 4 | 5 | 28 | 20 |

|---+----+----+------+------|
| 4 | 5 | 6 | 45 | 30 |
|---+----+----+------+------|
| 5 | 6 | 7 | 66 | 42 |
|---+----+----+------+------|
| 6 | 7 | 8 | ? | 56 |
|---+----+----+------+------|

Find out the number in "?"




Have fun!! (^v^)



p/s: TQVM eunice for forwading this email to me (^O^)