The operation of moving a variable from a register to memory is called spilling
So we will have to allocate only registers we can't store the result in memory
p=6 => lets save value of p in register R1
q=7 => since we need value of p in future instructions we cant save it in R1, so let us use R2 for q
t=p*q => now both p and q are used in future instructions, so we will have to store t in R3
s=t*p => p, q and t, all 3 are being used in future instructions so we will have to store s in R4
u=8 => p, q, t and s all 4 are being used in future instructions so we will have to store u in R5
u=s*p => no need of extra register. The operation is being performed on u that is R5
s=p+u => since we are updating value of s, again we dont need extra register this will be done in R4
r=r*q => now only t and p are being used in future instruction so we can store r in any of R2,R4 or R5. So no need of extra register
t=t+p => since we are updating value of t, again we dont need extra register this will be done in R3
Hence we need only 5 registers R1, R2, R3, R4 and R5
Hence answer should be 5
