Nancy has 26 marbles. Ted gives her 24 marbles. How many marbles does Nancy have now?

The correct answer is: 50

No. of marbles Nancy has = 26
No. of marbles Ted gave to Nancy = 24
Total marble Nancy has now = 26 + 24 (By partial sum)
To find partial sums add the digits in tens place and ones place seperately.
Partial sum of 26 + 24
Step 1 : Add digits in tens and ones place seperately:
20                                    06
+ 20                   and        + 04

_____                             ______
40                                        10
Thus, Total marble Nancy has = the sum of 26 + 24 are 40 + 10 = 50.