Answer: (a) 98,550Solution: Three-digit numbers divisible by 5 are 100, 105, 110, 115,.....995.These numbers are making an arithmetic progression series, hence $$ a = 100 $$ $$ d = 5 $$ $$ l = T_n = 995 $$ Let the number of terms in the series are "n" then. $$ T_n = a + (n - 1) \ d $$ $$ 995 = 100 + (n - 1) \ 5 $$ $$ 895 = (n - 1) \ 5 $$ $$ n - 1 = \frac{895}{5} $$ $$ n - 1 = 179 $$ $$ n = 180 $$ Hence the sum of all three-digit numbers divisible by 5. $$ S = \frac{n}{2} \ (a + l) $$ $$ = \frac{180}{2} \ (100 + 995) $$ $$ = 90 \times 1095 $$ $$ S = 98,550 $$ Learn Arithmetic Progression
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