How many such numbers are there between 10 and 1000?

Question

Many whole numbers between 10 and 1000 have 2 or 7 as the units digit. How many such numbers are there between 10 and 1000?

 

Answer 1

see....

from 10 to 20 there are 12 and 17 means 2 number

 

 

10 to 20 .....2 no.   (12,17)

20 to 30 .....2 no.   (22,27)

30 to 40......2 no.   (32,37)

40 to 50......2 no.    (42,47)

50 to 60......2 no.    (52,57)

60 to 70......2 no .    (62,67)

70 to 80......2 no.      (72,77)

80 to 90......2 no.      (82,87)

90 to 100.....2 no.      (92,97)

 

 

2+2+2+2+2+2+2+2+2=18

 

so between 10 to 100 there are 18 no.

 

between 100 to 200

 

100 to110.....2 no.(102,107)
110 to 120.....2 no.(112,117)
120 to 130......2 no.(122,127)
130 to 140......2 no.(132,137)
140 to 150......2 no.(142,147)
150 to 160......2 no.(152,157)
160 to 170......2 no.(162,167)
170 to 180......2 no.(172,177)
180 to 190.....2 no.(182,187)

190 to 200.....2 no.(192,197)

 

 

2+2+2+2+2+2+2+2+2+2=20

 

and so

 

 

 

100 to 200 there are 20 no

200 to 300 there are 20 no

300 to 400 there are 20 no

400 to 500 there are 20 no

500 to 600 there are 20 no

600 to 700 there are 20 no

700 to 800 there are 20 no

800 to 900 there are 20 no

900 to 1000 there are 20 no

 

 

18+20+20+20+20+20+20+20+20+20=198

 

 

so there are such 198 numbers