To find out how much the Turner family has saved after 15 months, you can multiply the amount they save each month by the number of months.
The calculation is as follows:
\[ \text{Total savings} = \text{Monthly savings} \times \text{Number of months} = 42 , \text{dollars} \times 15 \]
Calculating this gives:
\[ 42 \times 15 = 630 \]
So, the Turner family has saved $630 after 15 months.