I am trying to compute VAT field in my customized excel report, based on a total before vat field and multiplying it by 0.15 but everytime I execute the code I get errors like type error

below is the code

for invoice in invoice_objs:
    invoice_date = invoice.invoice_date.strftime('%Y-%m-%d')
    worksheet.write(row, 0, invoice_date)
    worksheet.write(row, 1, invoice.name)
    worksheet.write(row, 2, invoice.partner_id.name)
    worksheet.write(row, 3,  ', '.join(invoice.invoice_line_ids.mapped('product_id.name')))
    worksheet.write(row, 4, ', '.join(invoice.invoice_line_ids.mapped('tax_ids.name')))
    worksheet.write(row, 5, ', '.join(str(i) for i in invoice.invoice_line_ids.mapped('price_subtotal')))
    if 'Sales VAT' in invoice.invoice_line_ids.mapped('tax_ids.name'):
        worksheet.write(row, 6, '15%')
    else:
        worksheet.write(row, 6, '0%')
    if 'Sales VAT' in invoice.invoice_line_ids.mapped('tax_ids.name'):
    %%-->    worksheet.write(row, 7, (', '.join(str(i) for i in invoice.invoice_line_ids.mapped('price_subtotal')))* 0.15)

Please check the last line might be there is issue in that 

--------UPDATE-----------

Error Message: 

File "/home/suhaib/odoo/odoo/addons/invoice_summary_report/wizard/invoice_summary_report_wizard.py", line 93, in action_print_invoice_summary
    worksheet.write(row, 7, (', '.join(str(i) for i in invoice.invoice_line_ids.mapped('price_subtotal')))* 0.15)
TypeError: can't multiply sequence by non-int of type 'float'