Answer :
an easy way is to split it up or guess
so 15 times 2=30
18 times 2=36
19 times 2=38
20 times 2=40
so the answer is between 19 and 20
so the answer is 19.5
so 15 times 2=30
18 times 2=36
19 times 2=38
20 times 2=40
so the answer is between 19 and 20
so the answer is 19.5