Find a common multiplier then divide ex. 21/27 = 7/9
So the sides ML and MN are exactly the same. Your equation would look something like this...
3x = x + 10
You first subtract x from both sides to isolate the variable.
2x = 10
Divide 2 from both sides.
x = 5
5 is the length of MN.
I hope this helps love! :)
To work out the percentage you would do the amount he ate, divided by the original amount, and then you would multiply the answer by 100.
4 / 10 is 0.4
0.4 * 100 = 40
He ate 40% of the cookies.