Mathematics
posted by jessica meyz .
how many positive four digit integers have 3 as their tens digit and 5 or 9 as their ones digit?
i don't know where to begin

Start listing them. Fourdigit numbers must be between 1000 and 9999. The last two numbers must be 35 or 39.
1035
1039
1135
1139
...etc., ending with
9935
9939.
There are 90 pairs of numbers, starting with 10 and ending with 99 as the first two digits.
90 pairs = 180