Which mathematical statements are true?

1. incorrect 
odd number multiplied by odd number is still an odd number, 3 being an odd number when multiplied by 3 is 9 which is an odd number

2.incorrect
this states that if 6 is less than 7, which is true, then 4 is greater than 7, which is  incorrect. The 2 statements are not related

3.correct
6 is divisible by 3, 6/3 = 2, this means that 6 can be divided by 3 without giving a remainder.  
10 is a multiple of 2. multiples of 2 in other words are numbers that are divisible by 2. Since 2*5 = 10, its a multiple of 2.

4.Incorrect.
Average of data is the sum of the data values divided by the number of values there are, so the average value cannot be greater than the largest data value.
Also second statement too is wrong, there are some instances when the data values all may be equal e.g.: 45,45,45, then the average of these data are also 45, hence the average cannot always be lesser than the largest data value. Average should be equal to or less than the largest data value.

5. Incorrect
the slope is the rate of change m(gradient) = change in y/change in x. however the y intercept is not the initial value. Intercept is merely the point at which the graph cuts the y axis, not its initial value.

 6.incorrect
sum of the interior angles of a triangle is 180°. since there are 3 equal angles, 180°/3 = 60°. therefore each angle is 60°.