From 101 to 199 (except from 130 to 139) she needs: 1⋅9=9(sers)

From 130 to 139 she needs: 1⋅10+1=11(sers)

From 200 to 250 (except from 230 to 239) she needs: 1⋅4=4(sers)

From 230 to 239 she needs: 1⋅10+1=11(sers)

So she need to purchase: 11+4+11=26(sers)

ANSWER: 26 sers

10 + 20 + 30 + 40 + 50 + 60 + 70 + 80 + 90

= ( 10 + 90 ) + ( 20 + 80 ) + ( 30 + 70 ) + ( 40 + 60 ) + 50

= 100 + 100 + 100 + 100 + 50

= 100 . 4 + 50

= 400 + 50

= 450

There are meters of cloth clothes are :
1m5dm + 1m8dm = 3m2dm
Answer : 3m2dm

K = 1 + 3 + 7 + ... + 107

K = ( 21 − 1) + ( 22 − 1 ) + ( 23 − 1 ) + ... + ( 26 − 1 ) + 107

K = 1 + 3 + 7 + 15 + 31 + 63 + 107

K = 227