Error while making "Make File" unix

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C++

I have 7 files which are as follows:;
-hw1.cpp
-mul.cpp
-mul.h
-div.cpp
-div.h
-common.cpp
-common.h

Each and every file compiles without warning. and runs well... but when i make make file eg::: make hw1

following error is comming... Please help

shallin07@ubuntu:~/Downloads$ make hw1
g++     hw1.cpp   -o hw1
/tmp/cck5CENS.o: In function `main':
hw1.cpp:(.text+0x98): undefined reference to `IsDigitString(std::basic_string<char,>, std::allocator<char> >)'
hw1.cpp:(.text+0x152): undefined reference to `IsDigitString(std::basic_string<char,>, std::allocator<char> >)'
hw1.cpp:(.text+0x249): undefined reference to `multiply(std::basic_string<char,>, std::allocator<char> >, std::basic_string<char,>, std::allocator<char> >)'
hw1.cpp:(.text+0x30f): undefined reference to `divide(std::basic_string<char,>, std::allocator<char> >, std::basic_string<char,>, std::allocator<char> >)'
collect2: ld returned 1 exit status
make: *** [hw1] Error 1
shallin07@ubuntu:~/Downloads$ ;

Please help::