Fast Math Practice Flashcards
How to multiply by 25
Divide by 4 then multiply by 100
How to divide numbers with multiples of 4 by 4?
Left to right, digit by digit
Ex. 84/4 –> 8/4 =2 and 4/4 =1
concatenate to 21
How to divide numbers with non-multiples of 4 by 4?
I.e. 52/4
I.e. 52/4 –> decompose
- -> 40/4 = 10
- -> 12/4 = 3
- -> concatenate to 13
OR divide by 2 twice
52/2 = 26 –> 26/2 = 13
8*25
8÷ 4 = 2
2 *100 = 200
16*25
16/4 = 4 4*100 = 400
25*40
40/4 = 10 10*100 = 1000
24*25
24/4 = 6 6*100 = 600
25*60
60/4 = 15 15*100 = 1500
25*18
450
26*25
26/4 = 6.5 –> 6.5*100=
650
25*9
9/4 = 2.25 –> * 100 = 225
17*25
425
25*900
22,500
25*800
20,000
160*25
4,000
1,200*25
30,000
25 * 140
3500
Decomposition of
$32 x 25,000
$800,000
1/2 x 0.1
0.05
1/4 x 10%
.025%
Multiply single digit hundreds
E.g. 200 x 400
ten thousands
80,000
Multiple single digit thousands (5k x 3k)
millions
15M
Multiply single digit thousands by millions (e.g. 3M x 8k)
billions
24B
Percentage multiplication
Multiply two percentages which are each multiples of 10 (e.g. 10% x 20%) and express answer as percentage
(10% x 20% = 2%)
Single digit percentage multiplication and express answer as a percentage (e.g. 3% * 6%)
0.18%
1k x 1k =
1M
100 x 100
10,000
M x K
B
B / K
M
K x K
M
B x K
T
T / K
B
42B / 500
500 becomes 1/2k
Dividing by 1/2 is the same as multiplying by 2
42B divided by 1/2k is 84M
120M x 250
250 is 1/4 x 1,000
250 is 1/4 K
1/4K x 120M
Is 30B
