shrive555 wrote:
How many multiples of 5 are there between 81 and 358?
54
55
56
57
58
i tried this formula \(Sum = \frac{a1+an}{2}*n\)
How did you use this formula? We don't have the sum of all terms and we don't have n.
Focus on the logic here: Multiples of 5 between 81 and 358 are 85, 90, 95, 100.....350, 355
Question is: How many numbers are they?
85
90 = 85 + 1*5
95 = 85 + 2*5
100 = 85 + 3*5
.
.
.
355 = 85 + 54*5
Total 55 terms.
To get the 54 above, you would need to do the following: (355 - 85)/5
The point is, this is an arithmetic progression.
a, a + d, a + 2d, a + 3d ........ a + nd
Total number of terms = n + 1
The last term (355) is first term(85) plus n times the common difference(5).
355 = 85 + n*5
Solve this to get n as 54
Total number of terms = 54 + 1 = 55
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Karishma
Veritas Prep GMAT Instructor
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