pukkapine:
47,385 X 25
grid method is long winded - but splits it up into easier units for the child:
so 25 - split into 20 and 5
47,385 - split into 40,000 - 7,000 - 300 - 80 - 5
This is effectively splitting out the 'places' (ten thousands/ thousands/ hundred/ tens and units) - and then working out multiplications that are easily achieved because your either multiplying units (so recalling timestable facts) or multiplying tens+.
To be honest at this point (when numbers got bigger than 2/3 digits) I just found it faster to use vertical column method of multiplication are then just recall multiplication facts and remember to add '0' or '0s' in as you moved along from units to tens, etc...
Grid method:
Multiplying 5 x 40,000/ 7,000/ 300/ 80/ 5 (5 operations)
Multiplying 20 x 40,000/ 7,000/ 300/ 80/ 5 (5 operations)
Adding resulting products together: 5 operations (by place value - so adding ten thousands results/ thousand results/ etc...)
Adding all products together to recompose the number: 4 operations
TOTAL: 19 mathematical calcualtions or operations
Column method
10 multiplying operations - same as above in effect (but just multiplying single digits and carrying over if product >9 - effectively multiplying 5 x 5/ 5 x 8/ 5 x 3/ 5 x 7/ 5 x 4 - which all may seem easier if you know your times tables - then same idea with x2 (but putting in a place holder zero in the units column - first column on right under the 5 - resulting from 5 x 5)
5 additions: because your multiplying by 25 (two digits) there are two rows of digits to add up
15 operations.
Less work at this point to do it by column method - and if multiplication facts are sound - it can be quicker as well.
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I liked the coco44 answer - x100 and then divide by 4.
If your DC is too young to know how to divide yet - this can be thought of as x100/ halve (which = x50) and halve again (which = x25).
47,385 x 100 = 4,738,500 and this may be too tricky to divide in half and then 1/4 as a whole number
so split it up:
4,000,000 - 1/2 is 2,000,000 and 1/2 of that = 1,000,000
700,000 - 1/2 is 350,000 and 1/2 of that is 175,000
30,000 - 1/2 is 15,000 and 1/2 of that is 7,500
8000 - 1/2 is 4000 and 1/2 of that is 2000
500 - 1/2 of that is 250 and 1/2 of that is 125
add up 1,000,000 + 175,000 = 1,175,000
add 1,175,000 + 7,500 = 1,182,500
add 1,182,500 + 2000 = 1,184,500
add 1,184,500 + 125 = 1,184,625
(but again adding in a column is faster than horizontally mentally adding these 5 numbers).
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My view is that knowing these different methods doesn't hurt - sometimes knowing how to 'decompose' or 'deconstruct' large numbers becomes helpful - when you're dividing for example (like coco44 showed with knowing that 25 x 4 = 100 so multipling a number (say A) by 100 and dividing by 4 will give you 25 x that number A - but ultimately for speed - traditional column method will win out the larger the numbers are that you are working with.
HTH