let n number of nickels and d number of dimes
we have n+d=28 (1) and 0.05n+0.1d=1.95 (2) (one nickel = 0.05 of one dollar, and one dime =0.1 of one dollar) . from the (1) we get n=28-d plugging this value to (2) we get 0.05(28-d)+0.1d=1.95
multiplying both sides by 100 we get 5(28-d)+10d=195 --->>. 140-5d+10d=195. --->>> 5d=55 so d=11
from (1) we get n=28-11 = 17
so Jack and Betty have 17 nickels (and 11 dimes)