Answer:
31 (i.e 3 tens and 1 unit)
Step-by-step explanation:
217 can be split up as,
2 hundreds, 1 tens, and 7 units.
186 can be split up as,
1 hundred, 8 tens and 6 units
If we solve straight we'll have
(2-1)hundreds + (1-8)tens + (7-6)units.
(1-8)tens will give a negative value, complicating the operation.
To make our operation easier we can use the fact that 1 hundred is equal to 10 ten to further split the 217, and it becomes
1 hundred, 11 tens and 7 units.
The operation becomes
(1-1)hundreds + (11 - 8)tens + (7-6)units
= 0 hundred + 3tens + 1 unit
= 3 tens + 1unit
= (3 x 10) + 1 = 31
