At the assembly level, IDA reconstructs the instructions as seen in memory.
Constant multiplication adds an overhead of at least one additional instruction (like MUL or MLA for ARM CPUs); when a compiler detects two constants being multiplied in source code, the result is usually calculated at compile time and used as a single constant in the compiled binary.
If the compiler optimized your code by replacing
5 * 20 with
100, there isn't any way for IDA to disassemble or decompile it differently. In such a case that information has been lost during compilation and only exists in the application's source code.
Assuming ARM architecture and a little-endian CPU like TI MSP430, the first instruction,
MOV R1, #100
would produce the following opcodes:
If that is the case for you, you need to recompile the program with different compiler settings or use a different compiler to exclude compilation-time optimizations.
You can enable the view of opcodes in IDA with
Options/General/Disassembly → Number of opcode bytes (non-graph).