1
00:00:10,400 --> 00:00:15,933
we cannot factor out a 4 since 15 is not divisible by 4.

2
00:00:15,933 --> 00:00:38,800
we must factor by grouping. look for 2 numbers whose product equals the product of 4 and -15

3
00:00:38,800 --> 00:00:58,933
their sum needs to be equal to positive 4

4
00:00:58,933 --> 00:01:14,095
the first clue is the product of the numbers is negative, so a and b must have different signs

5
00:01:14,095 --> 00:01:16,333
but a + b is equal to 4

6
00:01:16,333 --> 00:02:01,133
so, the difference of their absolute values must equal 4

7
00:02:01,133 --> 00:02:39,467
let's think about all of the factors of -60

8
00:02:39,467 --> 00:03:46,733
6 band 10 look tantalizing since they are 4 apart

9
00:03:46,733 --> 00:04:06,735
we want them to add to positive 4, so larger number must be positive

10
00:04:06,735 --> 00:04:29,333
so, we want positive 10 and negative 6

11
00:04:29,333 --> 00:04:34,333
now, group first 2 terms

12
00:04:34,333 --> 00:05:06,800
now, group the second 2 terms. both are divisible by 5

13
00:05:06,800 --> 00:05:09,933
factor out a 2y

14
00:05:09,933 --> 99:59:59,999
now, factor out a 5