Let #m# be Mac's Mass

and convert the description a bit at a time into algebraic form:

5kg more than 3 times #color(green)("Mac's Mass")# is decreased by twice #color(green)("his mass")#, the result is 60kg

#rArr#5kg more than 3 times #color(green)(m)# is decreased by twice #color(green)(m)#, the result is 60kg

5kg more than #color(red)(3" times "m)# is decreased by #color(blue)(" twice "m)#, the result is 60kg

#rArr#5kg more than #color(red)(3m)# is decreased by #color(blue)(2m)#, the result is 60kg

#color(green)(5"kg more than "3m)# is decreased by #2m#, the result is 60kg

#rArr##color(green)(5+3m)# is decreased by #2m#, the result is 60kg

#color(red)((5+3m)" is decreased by "2m)#, the result is 60kg

#rArr#

#color(red)((5+3m)-2m)#, the result is 60kg

#((5+3m)-2m)##color(blue)(" the result is "60"kg")#

#rArr##((5+3m)-2m)##color(blue)(=60"kg")#

So we have:

#(5+3m)-2m = 60#

Combining the #m# terms on the left side:

#5+m = 60#

Subtracting #5# from both sides:

#m = 55#