Basic learning:
1+0.5=1.5
This can be written as a fraction:
1.5=121
This can also be written as:
121=1+21
Hence, question states:
99971+99972+99973+99974+99975+99976
This can be written as:
999+71+999+72+999+73+999+74+999+75+999+76
We can add the fractions separately. Also, 999 appears six times:
⇒6×999+(71+72+73+74+75+76)
⇒5994+(71+72+73+74+75+76)
⇒5994+721
⇒5994+3=5997