37.
Call the amount David has, d.
Call the amount Jhumpa has, j.
Jhumpa has $55.
Jhumpa has $21 more than David.
$55 is $21 more than David has, so if you add $21 to David's amount, d, you get Jhumpa's amount, j
j = d + 21
38.
Let the temperature at noon be t.
The temperature fell 16 deg from noon, so you must subtract 16 from the temperature at noon to get the temperature at 3 pm. The temperature at 3 pm was -3 deg.
That means that t minus 16 must equal -3.
t - 16 = -3