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gracie
How many three digit numbers are there whose first digit equals the sum of the second and third digits?

A. 18
B. 27
C. 36
D. 45
E. 54
Find a pattern, and start with numbers that don't have many addends.

After you write the first digit, write all the second digits in ascending order, then all the third digits in descending order.

101
110

202
211
220

303
312
321
330

404
413
422
431
440

The pattern is
100s have 2 such numbers
200s have 3 " "
300s have 4
400s have 5

900s will have 10

Total = sum of integers 2 - 10 inclusive
(9 groups, and each group has +1 more than its first digit))

SUM of consecutive integers?
(Average) * (# of terms, n)
Average = \(\frac{First+ Last}{2}\)

\(n=\)(Last-First) + 1 =
[(10 -2 ) + 1]= 9

(\(\frac{First+ Last}{2}*n)\)

SUM: \(\frac{2+10}{2}*9)=(6*9)=54\)

Answer E

:thumbup:

after the highlighted portion we can simply use Sn formula for athematic progression. Answer in 1 step.
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