For this case we can make the following rule of three:
21 ------> 70%
x --------> 100%
Clearing x we have:
x = (100/70) * (21)
Rewriting we have:
x = (10/7) * (21)
x = 30
Thus,
21 is 70% of 30
Answer:
21 is 70% of 30

Let's Solve ~





or


[ it's similar to expression - a² + 2ab + b² that is equal to (a + b)² ]
so, let's use this identity here to factorise :

I hope it was helpful ~
It's would be plus haft that number (it's a negative) and the next number would be 0