Присылай полный код, разберёмся.
aklin20@mail.ru

1)Поправленный код ( см. примечания в конце)


Private Sub C2_Click()

End Sub

Private Sub Command1_Click()
T1.Text = ""
T2.Text = ""
T3.Text = ""
T4.Text = ""
End Sub

Private Sub Command2_Click()
'T15.Text = T5.Text + T6.Text + T7.Text + T8.Text + T9.Text + T10.Text + T11.Text + T12.Text + T13.Text + T14.Text
On Error Resume Next
Dim i As Single
i = CSng(T5.Text)
i = i + CSng(T6.Text)
i = i + CSng(T7.Text)
i = i + CSng(T8.Text)
i = i + CSng(T9.Text)
i = i + CSng(T10.Text)
i = i + CSng(T11.Text)
i = i + CSng(T12.Text)
i = i + CSng(T13.Text)
i = i + CSng(T14.Text)
T15.Text = i
End Sub

Private Sub O1_Click()
If O1 = True Then T9.Text = 1 * 0.2
End Sub

Private Sub O10_Click()
If O10 = True Then T11.Text = 2 * 0.05
End Sub

Private Sub O11_Click()
If O11 = True Then T11.Text = 3 * 0.05
End Sub

Private Sub O12_Click()
If O12 = True Then T11.Text = 4 * 0.05
End Sub

Private Sub O13_Click()
If O13 = True Then T12.Text = 1 * 0.05
End Sub

Private Sub O14_Click()
If O14 = True Then T12.Text = 2 * 0.05
End Sub

Private Sub O15_Click()
If O15 = True Then T12.Text = 3 * 0.05
End Sub

Private Sub O16_Click()
If O16 = True Then T12.Text = 4 * 0.05
End Sub

Private Sub O17_Click()
If O17 = True Then T13.Text = 1 * 0.15
End Sub

Private Sub O18_Click()
If O18 = True Then T13.Text = 2 * 0.15
End Sub

Private Sub O19_Click()
If O19 = True Then T13.Text = 3 * 0.15
End Sub

Private Sub O2_Click()
If O2 = True Then T9.Text = 2 * 0.2
End Sub

Private Sub O20_Click()
If O20 = True Then T13.Text = 4 * 0.15
End Sub

Private Sub O21_Click()
If O21 = True Then T14.Text = 1 * 0.2
End Sub

Private Sub O22_Click()
If O22 = True Then T14.Text = 2 * 0.2
End Sub

Private Sub O23_Click()
If O23 = True Then T14.Text = 3 * 0.2
End Sub

Private Sub O24_Click()
If O24 = True Then T14.Text = 4 * 0.2
End Sub

Private Sub O3_Click()
If O3 = True Then T9.Text = 3 * 0.2
End Sub

Private Sub O4_Click()
If O4 = True Then T9.Text = 4 * 0.2
End Sub

Private Sub O5_Click()
If O5 = True Then T10.Text = 1 * 0.05
End Sub

Private Sub O6_Click()
If O6 = True Then T10.Text = 2 * 0.05
End Sub

Private Sub O7_Click()
If O7 = True Then T10.Text = 3 * 0.05
End Sub

Private Sub O8_Click()
If O8 = True Then T10.Text = 4 * 0.05
End Sub

Private Sub O9_Click()
If O9 = True Then T11.Text = 1 * 0.05
End Sub

Private Sub T1_Change()
On Error Resume Next
If T1.Text < 10 Then T5.Text = 1 * 0.15 Else
If T1.Text >= 10 And T1.Text < 20 Then T5.Text = 2 * 0.15 Else
If T1.Text >= 20 And T1.Text < 30 Then T5.Text = 3 * 0.15 Else
If T1.Text >= 30 And T1.Text < 40 Then T5.Text = 4 * 0.15 Else
If T1.Text >= 40 Then T5.Text = 5 * 0.15
End Sub


Private Sub T2_Change()
On Error Resume Next
If T2.Text < 70 Then T6.Text = 1 * 0.05 Else
If T2.Text >= 70 And T2.Text < 80 Then T6.Text = 2 * 0.05 Else
If T2.Text >= 80 And T2.Text < 90 Then T6.Text = 3 * 0.05 Else
If T2.Text >= 90 And T2.Text < 100 Then T6.Text = 4 * 0.05 Else
If T2.Text >= 100 Then T6.Text = 5 * 0.05
End Sub

Private Sub T3_Change()
On Error Resume Next
If T3.Text < 70 Then T7.Text = 1 * 0.05 Else
If T3.Text >= 70 And T3.Text < 80 Then T7.Text = 2 * 0.05 Else
If T3.Text >= 80 And T3.Text < 90 Then T7.Text = 3 * 0.05 Else
If T3.Text >= 90 And T3.Text < 100 Then T7.Text = 4 * 0.05 Else
If T3.Text >= 100 Then T7.Text = 5 * 0.05

End Sub

Private Sub T4_Change()
On Error Resume Next
If T4.Text < 70 Then T8.Text = 1 * 0.05 Else
If T4.Text >= 70 And T4.Text < 80 Then T8.Text = 2 * 0.05 Else
If T4.Text >= 80 And T4.Text < 90 Then T8.Text = 3 * 0.05 Else
If T4.Text >= 90 And T4.Text < 100 Then T8.Text = 4 * 0.05 Else
If T4.Text >= 100 Then T8.Text = 5 * 0.05

End Sub

2)Пояснения
Csng=Представить в типе Single( у тебя представлялось в String)
On error resume next = Даже если возникнет ошибка, пропусти и иди дальше.
3)
Удали T15 и поставь заново.
4) в принципе всё-равно как делать в Command2_Click: через i или напрямую.