let

x = the number of pennies that 2nd grade collected

y = the number of pennies that 5th grade collected

the 2nd grade collection 3 times fewer pennies than the 5th grade:

x = y/3 divide both sides by 3

y = 3x

the 2 and grade collected 1845 pennies:

x = 1845

plug x = 1845 into y = 3x

y = 3*1845

y = 5,535 pennies

the 5th grade collected 5,535 pennies.